How i can know if the element that is not working well is the starter  or the tube  ?

Replace one or the other (not both) with a known working one.

How it behaves?

In any way, unless the things are really an unobtanium, I would recommend to replace both. The "don't replace what is not broken" does not apply here, as the things are broken even when it is not visible yet.

The reason is, if one of them goes bad (it ma still seem to do its job though, just starting to collapse), it destroys the other as well.
Most often the first to die is the tube. The last hours before its final breath it starts to exhibit elevated reignition pikes, which cause tye glow in the starter to keep warming the starter, but not enough for the contacts to actually close. This causes first heavy sputtering there, so the parameters (mainly the trigger voltage) shift. Second it heats up the whole assembly, slowly cooking its components. Only after some time, So far you do not notice anything from the outside, the lamp still lights normally, but the starter parts become already pretty toast.
As the things are deteriorating, at one point the starter starts to cycle and only then you may spot the failure. If you replace just the tube, the new one ma seem to work, so you think you have replaced the bad component. But what you don't see is the starter on the verge of collapse, with its timing and voltage completely off. As the new tube wears a bit (but still early in its life), the bad starter starts acting again. Often replacing the tubd may seem to "solve" the problem yet again. This may repeat even few times, it is just you then complaining about the "bad new tube quality compare to the good old starter". Then at one point the old starter welds its contacts shut, keeps overheating the filaments, eating away most of the lamp life really within few hours. You replace the starter, everything seems to work again, just the damaged tbe fails in a short time, again damaging the practically new starter. And wash, rinse, repeat, with a lot of cussing about "the modern components don' last as they used to".
But all that is just because "I won't replace what isn't broken"...

If you compare cost (and environmental impact) of replacing the lamp vs the starter, mainly when it failed after really short time and you cought it early on (so there is a chance it was a deffective starter with not much damage to the lamp yet; or it was a starter opencircuit defect) you may try to replace the starter first and see if it won't start working, so you get some extra working time out of the more expensive (to purvhase, but as well the environmental impact of disposing of the old one), but very likely you have just sacrificed one extra starter for the chance of getting the full life of the lamp. But if the thing was turned on with "stuck starter" for longer than few minutes, even when it started as a failed starter alone, very likely the tube got destroyed as well.

Yes, I agree with Medved here, even the GE lamp guide says to replace BOTH tube and starter at a re-lamp, however I’ve had other faults inside switch start battens such as loose wiring in the lamp holders ballasts and even starter sockets!
It is rare if your new tube and starter don’t work when installed, but it can happen.

Thanks for all the info, now i know all the process when a lamp starts to EOL

Thanks for all the info, now i know all the process when a lamp starts to EOL

Combined with my personal experience: if the lamp keeps flashing and repeating the start action when the power is on, it is that the electronic powder loss on the lamp filament is too large, which makes it difficult for the lamp to maintain discharge. The lamp needs to be replaced. If the lamp does not blink when energized and the two ends (at the filament) continue to emit light, the starter contacts are stuck, difficult to move or leak, and the starter needs to be replaced.

@bulb_tester2009

But the point is, even when the starter seems to be still working (the flashing tube situation), it is already heavily damaged, so it will destroy the new tube within rather short time. This is the reason, why you need to replace both, even when the starter still "is OK".
And not letting the tube to flash won't help, as the majority of the damage occurs before the tube starts flashing, what you as the user can never spot in time, so once the tube starts flashing, the damage is already done.

You may use the thermal cutout starters, they trigger the bimetal cutout immediately as the heating starts, but still these are rated to last just 3 tubes. And that assume you won't be trying to "just reset it" without actually replacing the tube. It may lead to seemingly successful start, but the triggered cutout means the lamp has reached the state where it starts damaging the starter. And because still some damage happens before the cut out trips, the starter lifetime is still limited to just 3 tubes. Exceeding that and it will start damaging the tubes too.
Given their cost (>5x the normal starter) and the need to keep records about how many tubes has each individual starter "on its belt", I don't see much of their use unless the aim is to prevent the annoying flashing of EOL tubes in installations with scheduled relamping schemes (so which are intentionally operated with few dead tubes).

But then I would rather go for the electronic starters - these really operate the lamps till they are able to start, yet are not flashing or overheating anything (they just have a timer cutout, usually about 2..3s or so for starting attempts) and normally do not wear out that fast (their lifetime is 10's of lamps, so pretty much the fixture lifetime). Plus they operate the lamps so the starting wear is very limited, so even making the lamps last long.

@bulb_tester2009

But the point is, even when the starter seems to be still working (the flashing tube situation), it is already heavily damaged, so it will destroy the new tube within rather short time. This is the reason, why you need to replace both, even when the starter still "is OK".
And not letting the tube to flash won't help, as the majority of the damage occurs before the tube starts flashing, what you as the user can never spot in time, so once the tube starts flashing, the damage is already done.

You may use the thermal cutout starters, they trigger the bimetal cutout immediately as the heating starts, but still these are rated to last just 3 tubes. And that assume you won't be trying to "just reset it" without actually replacing the tube. It may lead to seemingly successful start, but the triggered cutout means the lamp has reached the state where it starts damaging the starter. And because still some damage happens before the cut out trips, the starter lifetime is still limited to just 3 tubes. Exceeding that and it will start damaging the tubes too.
Given their cost (>5x the normal starter) and the need to keep records about how many tubes has each individual starter "on its belt", I don't see much of their use unless the aim is to prevent the annoying flashing of EOL tubes in installations with scheduled relamping schemes (so which are intentionally operated with few dead tubes).

But then I would rather go for the electronic starters - these really operate the lamps till they are able to start, yet are not flashing or overheating anything (they just have a timer cutout, usually about 2..3s or so for starting attempts) and normally do not wear out that fast (their lifetime is 10's of lamps, so pretty much the fixture lifetime). Plus they operate the lamps so the starting wear is very limited, so even making the lamps last long.

Thanks for the explanation. But in China, high-quality electronic starters are very rare. So we can only use traditional starters. For traditional starters, I check the blackness of the starter when I change the lamp to determine if I need to replace the starter. But I'm sure most users won't do that.

I have a few fixtures that only work if the tubes and starters are in a certain orientation.