I've seen madvapes juice advertised on their site as USP:
PROPYLENE GLYCOL 4 oz
Not sure about their premixed flavors.
I've been using Humco VG from Walmart for my DIY e-juice. States on the bottle Glycerin, USP. I've read that others don't trust this brand, but read a post on ECF from a pharmicist saying it was fine, plus I've been vaping it for quite awhile now with no adverse effects.
I suppose the best way to proceed would be to contact the supplier by email or phone and get the info directly.
Email probably best, since then you have it in "writing".
Good luck in your future vaping endeavors!