Region Locking: Best Way to Determine?

Hello,

If it is from a bundle search the bundle name in the deals discussion tab and you will find out details about it including possible region-locks, thanks to sensualshakti and community contribution.

If it's from a bundle, you can check https://www.steamgifts.com/discussions/deals then search your bundle's name.

You can also check the sub yourself by using https://steamdb.info/extension/ on your browser then activate the key at https://store.steampowered.com/account/registerkey but first, make sure that you've already owned the game in your account. Using the extension, it'll give you a SteamDB link containing details about what game you're trying to activate along with its restriction

on top of what others have said you can just check steamDB and look to see what region locks match what fanatical says.
with mystery bundle keys if you ever see PRC restricted on steamDB that is likely the sub.