How can you tell if a rash is caused by an allergic reaction or something else?

Can't: There's an old saying in dermatology: you can't diagnosis a rash over the phone. I even get confused after seeing some rashes. Generally, allergic rashes are welts but some allergic rashes are small red dots. Best to see your dermatologist.

Rash: You really need to have a doctor look at it to determine the difference. Allergic rashes tend to be short-lived and might be a simple hive appearance to it.