I don't know but the first thing that came to mind was my school training. We tested fabrics by burning them to see what they would do. Different fabrics react differently.
You could Google the fire test for poly and rayon.

Polyester burns and melts leaving a hard ball, rayon burns like cotton and leaves some white ash. The thin thread burns rapidly in both cases so use tweezers.

Polyester thread also has more stretch to it than rayon.

This is good to know. Now to find a place to keep it in my "memory bank". I did give it an orange tick so it will be easier to find.

Don't trust my memory bank-Going to write this one down in my 'little book'

Some thread brands use a different color spools for poly and rayon.

Great info here. I know they say 40 wt , but I always think the rayon is fuller or larger looking. Of cource the sheen in some brands is a dead give a way.Too bad they don't makebetter stickers for the spools. I have even tried to tape some on the spool. Crazy!!

I needed this info badly. Will have a fire tomorrow.
I have spools that I haven't a clue as to what they are. In the past we had a few places here that sold
spools of thread that came from Vanity Fair before
they moved to Mexico. Maureen

I've taken to writing the number & brand name inside the reels with a cd marking pen but with dark colour reels I had to make do with white price stickers with the details written on them .I just hope they don't peel off.Some brands stupidly put all the info on top of the reels so when they are put onto the machine the reel holder pokes through & everythin is lost.
I am going to do some 3D flowers on organza & I've been told to use rayon thread ,so when I use the stencil burner to go round the edges of the flowers the thread won't burn.