If you know the voltage(easy) and check the ohms(easy) you know the amps. voltage/ohms = amps. Ohms will probably be below 1.
Watts are amps x volts fyi. Easy to check how many amps a light will pull. If you buy a 50 watt light and you know with the car running you have 13.4 volts - 50/13.4 = 3.74amps. If the car was off and you only had 12 volts it would be more amperage 50/12 = 4.1amps.
With my 4, 55 watt (wussy) offroad lights on my rack the amperage is (not including resistance from the length of 10gauge wire) (55w/13.4v)x4 = 16.41 with the engine on and (55w/12v)x4 = 18.33 with the engine off.