Scent of New Mown Hay
Good advice from someone who knows:
Smell new mown hay, green corn, or something fruity? Probably a nerve agent. Breathe shallowly, head upwind.
Smell bitter almonds, garlic? Probably a blood agent like cyanide or arsenic. Have your poppers (amyl nitride) ready and snort like crazy while heading upwind. This is probably good advice for all occasions actually.