How to tell when water reaches 100 degrees Fahrenheit without a candy thermometer for the purposes of disolving gelatin?
Gelatin is quite tolerant, but with a few restrictions:


Never boil gelatine, because it looses it's binding/gelling properties.
Liquifying gelatine requires temperatures that feel "warm" to the touch, but not all recipes handle warm additions well - e.g. whipped cream.
Cooling liquid gelatine for heat sensitive recipes should happen fairly quick, and so should incorporating the coolish gelatine to the other ingredients : Stir well or you end up with "gummy bear"- like lumps.


So without knowing the details of your recipe I suggest using fairly warm water in the range of "warm bath water" or "comfortable for washing hands" but not at all "hot". (Note that I don't give a precise temperature range on purpose.)  Hot tap water should suffice, but heating it on the stove is fine, too. Just use gentle heat and don't let the gelatine rest on the bottom of the pot to avoid overheating.
Depending on your next steps, you might want to stir the liquid  gelatine until it's barely warm to the touch or add a few spoonfulls of whatever cool mixture you are planning to bind, stir, then add to the rest. For warm other ingredients, extra cooling is not required, just mix and let set.