I bought the one on Ebay. I hope that seller is true to his word and they are 100% wool. I'm glad to be looking for them now with hard they are to find.
Well, you could test it. Trim a small amount of one end ( neatly, no point ruining it entirely), and see how it behaves with fire.
It should be very difficult to get lit, burn poorly ( dull or no flame, *black* smoke) and self extinguish rapidly. ( it may not light at all, that is still ok, as it is retained lanolin that'd be burning, and that doesn't last long)
What it should not do, under any conditions, is burn well, bubble, melt, bead or edge soften. if it does any of those, it has a substantial adulteration.
To detect small adulterations you do need a dissection microscope however
Edit: this of course assumed it has been well washed to get rid of any fire suppression coating (they are useful if you know that it's there and what it's covering, but i would never trust such a coating on its own)