Observations:
When you cut 100% poly fabric, the threads on the cut edge are straight, no bend to them. The fabric usually doesn't wrinkle. I think it is harder to piece with, so generally don't use it. Sometimes it pills. Generally, it seems to ignore the pressing to the dark side thing.
In my opinion, it doesn't quilt up nearly as well as a cotton or a good poly cotton. As someone already suggested, a hot iron will tell you whether a fabric is poly or cotton. It melts poly. The melt stays on your iron, so I suggest that you put a piece of freezer paper or aluminum foil over the test fabric before you press. Alternatively, You could cut off a small piece of the fabric, place it in a bowl and light it with a match. The residue is not ashy like cotton, but looks like a small twisted piece of black plastic. Best to do that near a sink so you can douse the fabric piece quickly if you need to. (To date, I have never needed to!)
Cotton, on the other hand, the threads are softer, and will feel like a cotton thread, and definitely will bend and curve. It pieces more smoothly, and obeys the iron.
Poly/cotton usually does not wrinkle like a 100% cotton fabric. It is less likely to obey the iron than 100% cotton, but generally can be pressed into submission. I have never flame or iron tested poly cotton. The threads at the cut end are generally straighter and hold their shape more than 100% cotton, but have more bend than 100% poly.
There are different percentages of poly/cotton, 70/30, 60/40 40/60 and so on, so you really have to test piece the poly/cottons to see if they will work like you want when piecing with it. I use this occasionally as backing, but generally not in when piecing a top.
Because of the melt factor, NEVER use poly or poly/cotton in things like pot holders, table toppers, etc. That is also true for polyester or poly/cotton batting.