Look at the surface
THHN has a thin, slick nylon jacket over PVC insulation, so it feels glossy and slides through conduit. XHHW is cross-linked polyethylene, usually matte and slightly waxy, sometimes with a nylon jacket of its own on XHHW-2 constructions.
Read the print
THHN, THWN and THWN-2 are printed in full, often with the temperature ratings. XHHW appears as XHHW or XHHW-2. Where both appear on one legend, the wire carries both ratings and prices accordingly.
Strip a short end
Thermoplastic insulation cuts cleanly and shows a sharp edge. Cross-linked polyethylene is tougher and tends to deform rather than slice. Do not strip a good reel to check — a cut end that already exists is enough.
Why it matters
XHHW carries a wet-location rating and better moisture performance, which usually makes a reel easier to place in plant and underground work. THHN of the same size may sit longer, and that difference shows up in the offer.