I think a viral rash is a spotty rash. When my kids get these sort of rashes, I don't really worry too much.
Little seperate red spots on the chest &/or tummy.
If there's a whole red area, that's lumpy, all red & itchy I'd have it looked at. Could be eczema, or an allergy.
Any sort of purple rash needs to be looked at too. I'm sure you'd know that though.

Hope those descriptions made sence.