It used to be possible by examining the items, but they removed that, I don't know why. Anyway, members items can go in F2P slots, and probably the opposite is true.
Bank boosters can be used in F2P if you need more space, and by getting members once you can claim the free bank space boosters from Solomons, then if you want go back to F2P and you will keep those addition bank spaces you got from becoming a member.Hi.
Kings Abbotsaid: The colour of the name of the item shows if it's free-to-play or members only. Light blue is f2p, orange members I believe. That, or withdraw the (stack of) item(s) and see which counter goes down.
Oh I didn't notice that was still there. So if you hover your mouse over an item the text color says if it is a members item or F2P.