You should clean the floor around where it normally leaks first. The next time it leaks soak some up with a white paper towel or napkin. You should be able to identify if it is coolant or normal water. Coolant will have color (typically orange or green) and smell. You may also notice the coolant level dropping. If you're not comfortable working on your truck, it would be a good idea to take it to a good shop if you determine that it is, in fact, coolant.
If it's normal water, it could be condensation off the AC compressor finding its way to the ground. Many vehicles activate the compressor with the "defrost" function.