Although I don't like to spend a lot of time on this sort of thing in-game, this is actually a good place for languages or knowledge of history to come into play. If you label the bottle in a language other than Common, PCs with the corresponding language proficiency (or a comprehend languages spell) can figure out what it is. You might also describe the potion bottle as having been designed in a particular way or perhaps it bears a heraldic symbol. PCs trying to recall lore on such designs or symbols might be able to figure out what the potion is.