I have a good friend (former teacher) tell me that a good way you can tell if your multivitamin actually dissolves in the stomach was to test it in a dish of vinegar, since the acidity of vinegar is the closest to stomach acid.

I remember her telling me, that when her student did this experiment, a Centrum multivitamin never dissolved, and actually performed the worst out of the other vitamins she tested.

Try this experiment with your own vitamins to see if yours is working.