Split out some of the warp yarns and weft yarns from two kinds of webbing which you don't know the material; respectively, burned with a lighter, observation of physical phenomena in the combustion process to determine raw material for weft yarn. Ovserve the condition of the burning flame, melts, the smell, ash when burned.
Here are some physical properties of two common materials of webbing, for your reference:
Nylon webbing: Polyamide (nylon) with scientific name of polyamide fiber, rapidly rolled melt white gel when near the fire, molten fuel dripping and bubbling in the flame, no flame when burning, hard to continue burn when leaving the flame, emitting a celery flavor, with a light brown melt not easy to crushed after cool.
Polyester webbing: with scientific name of polyester fiber, flammable, rapidly rolled melting when near the fire, melting and emitting black smoke when burning, with yellow flames, smells odors, the ash is dark brown lumps after burned, can be twist to pieces with your fingers, have black ash around the webbing.
Seat Belt Webbing