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.

You are not specifying whether it's 100 degrees Celsius or Farenheit.

If it's Celsius, 100°C is the temperature of boiling water (at sea level). Just full boil it.

If it's Farenheit, 100°F is very close to body temperature (if you're not ill). You can use a normal medical thermometer.

Besides the usual measuring cups and a tongue for licking the cake batter bowel, you should have a digital scale and a thermometer as standard "kit" (as the Brits would say). You don't need a high temp candy thermometer for most cooking. There are some thermometers that you can put in the oven and some that you can't...they will literally melt.

You need a general thermometer for food safety. I recommend a probe thermometer.

In a pinch, if you have a window thermometer, the glass bulb kind, you might be able to clean it up a bit and use that. The newer ones don't contain mercury. They have alcohol with a dye. So I would consider it safe to use.

Likewise, if you have a medical thermometer in your medicine chest, the 98.6 thermometers used for humans is in the ballpark of the 100F degrees you're looking for. A digital medical thermometer would be even better as they give you an accurate reading in a split second.