Check the information printed on the bulb or the bulb's packaging to find the efficiency rating. This can be anywhere from 5 to 15 lumens per watt for incandescent bulbs or 70 to 300 lumens per watt for LED bulbs.
Check the packaging for the number of watts used by the bulb. For an LED light bulb, make sure to calculate with the number of watts used and not the number of watts replaced.
Multiply the efficacy rating by the watts used. The answer is the lumens or brightness output for that light source.