You can only use them in battle - select the use item option, then select the potion you want to use and it'll ask for a target and count like spells.
Just tried it in game, and it's throwing an error - I guess I never actually tried to use potions XD. I've made a bunch of changes to the original game (and the potions are already fixed there) - so I'll transfer those over and update revenant once I'm done with the Stardander demo (have to get it ready for the Steam Next Festival).