See this interesting article from qz.com:
Nymphs, though, are roughly half the size of adults, about the size of a poppy seed. Tiny enough to pass for a smallish mole or freckle, they’re far harder to find than adults are; no surprise, then, that most human Lyme infections come from nymphs. All you can do is to take precautions against getting bitten, search for ticks thoroughly, remove them carefully if you find them, and look out for Lyme symptoms afterwards. Here’s advice on how to do all those things.