How can you tell the difference between jock rash and herpes? I had athletes foot then got the rash on the inner thighs

Most likely . Jock itch. It is the same fungus that causes the break out. Jock itch usually has flat, red, scaling rash vs herpes that has break outs, "hive like", lesions. If you have had unprotected sex, then have a gyn doctor or your primary care physician evaluate you to make sure you do not have herpes.
Tines cruris. The rashes look different. Definitive diagnosis is made by culturing a scraping for fungus . This will also tell you what type of fungus it is because different ones will have different characteristics. Viral culture of a vesicle will prove whether herpes simplex is present. Put your socks on before your underpants. Many times the underpants touch your toes and bring fungus into your groin.