if infinate resistance, the bulb is bad., good bulbs read fairly low resistance.,

 

hi,
Usually the working voltage and wattage are marked on the bulb/lamp

Using the formula Resistance = (Volt* Volt)/Watts will give the HOT resistance of the lamp when its operating normally.
The COLD resistance is approx 10 times less.

So a 12Volt , 1Watt lamp would have a HOT resistance of 144 Ohms and its COLD resistance approx 14 Ohms.

E

 

If it's an incandescent bulb, it should read a few ohms. A fuse will basically read zero. Bulbs don't fail as shorts, in my experience (changed a lot of them in the days of pinball machines), so if you see zero ohms, it's sure to be a fuse. A bad bulb will read infinite ohms, same as a blown fuse, same as nothing connected at all.