So, how can we tell which lamp type is used in the roadsters?
The HPS is easy enough because it'll be orange. But what about the MV & MH ones? I believe the MV gets a bluish to greenish tinge with age but both the MV & MH are white when new.

Only by characteristic lines in the spectrum.

Or if there is some identification system in place like NEMA-tags, the lamp type is color coded in the tag (and the wattage is in the number).

Only by characteristic lines in the spectrum.

Or if there is some identification system in place like NEMA-tags, the lamp type is color coded in the tag (and the wattage is in the number).

Hmm interesting. So I'd need a spectrometer? How much would that cost roughly?

I've noticed there are serial numbers on only some of them

A very simple spectrum analyzer is a CD or DVD. You look at the reflected light at an angle, where it shows the "rainbow" effect. It is nothing else than just the light diffracted according to the wavelengths, so most of the spectrum features you will see just by such simple tool by just an eye.
On the web are many "how-to's" to make an actual spectrum analyzers made of just a cardboard, using a cut out from a DVD or CD as the diffraction grating, if you want something better (most of them describe, how to tie them onto some webcam or attach to a phone camera, some present even links to some spectrum analyzing software).

For the NEMA tags, just browse through road lantern pictures of the US or Canada based members here, most of them do feature that label...

Besides diffraction gratings made from old CD's or DVD's you can also use a prism-Often available from science suppliers or taken from an old pair of binoculars.Project the light from the source you want to test thru the prism onto a white paper screen.Beleive Edmund Optical is still around-you can get a prism from them.

I don't think the prisms from binoculars will work, they are especially engineered to suppress the effect as much as possible.

But if speaking about special supplies, there are real diffraction grating sheeds sold around, mostly targeted for making laser effects (you would be looking for one advertised to make a series of dots) they are way superior to the DVD's or so. But not that readily available as the CD's or DVD's.

Oh yes,thats a point-the Poro prisms in binoculars probably won't work-but if you have one removed from the binocular-worth a try.Also Edmund sold diffraction gratings that were designed for spectroscope work.The prisms that were surplus came from tank periscopes.I used to have a diffraction grating spectroscope that came with a science set I had while I was a child-now wished I had it-would be useful in analyzing light sources.Lab type scopes are too expensive for the common guy.I also had a prism in those early days-think it got lost or thrown away during the long course of time.Another tool that would be helpful to a "flashaholic".