How can I tell if my son has heat rash or chicked pox?

Location. Chicken pox should be spread over the entire body whereas a heat rash will only occur where the skin became too hot/moist.
Chicken pox Rash. Whereas chickenpox is a viral illness, you should expect some viral symptoms like fever. The rash is a lot nastier looking also, with crusting from old lesions and some oozing from fresher ones. Often described in medical literature as having a "dewdrop on a rose petal" appearance. Heat rash is - well - just a regular bumpy rash.