Do Jet-A and 100LL smell different?
If you know what you're looking (sniffing) for, yes.

Avgas 100LL smells like gasoline (a little different than what you pump into your car, but similar).

Jet-A smells like kerosene (not quite "like diesel", but definitely not like gasoline).

If you are familiar with how both smell you can tell them apart by odor, and a significant contamination of Avgas with Jet-A could be detected by smell. (Other tests, like an evaporation test, are more reliable for lower levels of contamination however).