Usually, the tests look for cotinine, not nicotine. It can show up in a blood or urine test. Cotinine is more stable and lasts longer in your body, and the only reason you’d have it in your body is from nicotine.
When you’re using a nicotine replacement product, you may need a test that looks for nicotine, cotinine, and anabasine, a substance that’s found in tobacco but not in nicotine replacement products. If you test positive -- anabasine is present in your body -- that means you’re still using tobacco. Otherwise, it wouldn’t show up.