What to do if i can't tell if i have heat rash or an allergic reaction. help!?

Better description: A heat rash is an unspecific term but presumably related to heat. How long does it stay with you after the exposure and whether it itches or not is another issue we need to know. Also we need to know the time of onset of the rash after sun exposure. In general , an allergic reaction is often accompanied by itching. Regardless, the prudent answer is avoid the exposure. Sunscreen tried yet?

Antihistamine: As long as you have no other symptoms beside rash and itching (like breathing difficulties or the like) you could try some over the counter antihistamine, like Allegra (fexofenadine) or benedryl, and see if things improve. If it persists, ask your doctor about it. Allergies and heat rash typically look very different.