I have this Mercury Vapor fixture that is totally unmarked, except for General Electric in the socket.  I don't know what wattage it is supposed to be.  Does anyone know of a good way to find out?

I did one small test that is leading me to believe that it's 100 watt. I tried three MV lamps in it, a 100 watt, an 175 watt and a 250 watt.  I put a digital volt meter across the contacts of the socket.  With the 100 watt lamp, the output voltage stabilized at approx 138 volts.  With the 175 and 250 watt lamps, the output voltage stabilized at approx 88 volts.

I looked here in LampTech. It's 100w ballast for sure. Got a pic of that fixture?

Unfortunately I don't, and it will be a while until I can get one.  I just didn't have time to do much collection photographing last time I was home.