What chemicals can be detected in sweat?
Cystic fibrosis diagnosis. Here is a link to a paper describing the use of β-adrenergic sweat secretion as a diagnostic test for cystic fibrosis. 

Anti-cancer therapy and its effectiveness. This report seeks to determine the levels of the anti-cancer drug sunitinib and its active metabolite N-desethylsunitinib in sweat.

I'll add others later.

Here's a 2004 study looking at a particularly unpleasant sulfury compound produced by bacteria in contact with sweat. It's interesting that the enantiomer of the malodorous compound had a grapefruit-like smell.

This paper puts forth the idea that sweat, which contains lactate, lactic acid, urea (of course) and amino acids, can be used in forensics.