Can you tell by smell alone if someone is ill? And if yes, what is the scent that you are smelling?

I can't but I think my dog can sense when I'm sick.

I can usually tell if someone has a sinus infection, as some people develop what I call the "snot smell" just a particular smell that is difficult to describe!

NO ... A smell may be an indication of sickness, but we can't tell by smell alone.  It could be sickness, bad hygiene, something they ate or came in contact with, etc.

A fruity breath can be diabetes.