COM 201 BIG BEN
Since there are three (3) switches outside the toilet, which I assume are for three different lights, and one of the switches controls the light bulb inside the toilet and it is in the off position for a start, then:
1. The particular switch of the three (3) will be in a different position from the rest, so that if it is depressed, there would be no other light put on or put off on the outside of the toilet, and then I can be sure without a doubt that, that switch controls the toilet bulb.
2. If all the switches are in the same position, then I'll just depress each of the switches and notice the one which does not put off or put on any lights outside the toilet, and I can be certain that, that is the one for the bulb inside the toilet.