Get one of these babies for less than the price of 2 grams!
These were sold in the 70s and 80s, and still show up on auction sites. Here's a photo of a real one.
Melting point is a range, not a single temperatures. Very small amounts of fentanyl aren't going to change that range enough to really register. You would need to use another test, probably a chemical reagent, to check for fentanyl. Or send your drugs out to be tested by high-pressure liquid chromatography and mass spectrometry.