I would probably just buy more batting so I could be sure.
I suppose you could try a burn test on anything that is low loft & doesn't have a scrim (a scrim is when one side is smoother than the other). If it burns & only turns to ash, with no melty bits & no chemical smell, it's most likely cotton. That said, having worked years in the insurance industry & seeing what happens when people are "pretty sure" something was microwave safe, I really wouldn't risk it. They sell packages of "microwave-safe" batting (assuming you only plan to heat in 30 second increments for no longer than 2-3 minutes) at JAF. Since you only need the Craft Size & they have coupons available on their website for 40% off, I'd just get something where I knew it was 100% cotton with no chemical additives (e.g., bonding or bleaching agents).