I was surprised how easy it was for me to tell. When I opened the pack, it was in the middle of the stack and upside down. The back is glossy and the front has a dull surface.
I did the same thing. No luck. Remember the front is like older Topps cards that have no gloss. The back is the glossy smooth side. Good luck! I'm addicted to this set.
I just checked them on Ebay. At least half of them are not flip stock. This listing has good pictures of what it should look like https://www.ebay.com/itm/254956239109?hash=item3b5c937905:g:YEoAAOSwSCFghgXQ I almost bought a Bryce Harper that was listed as a reverse team color swap card. I checked my Harper and other Phillies. It wasn't a color swap card.