Given that pokemon seem to be able to at least understand humans even if they can't be understood back, couldn't someone just say "Hey can you show me what you can do?" and have the pokemon display its moves? Or ask one of the pokemon they already have to ask. If they couldn't categorize all of them, they might be able to place some and make guesses at the others. They could probably still have them use a move even if they don't know the appropriate name for it. Like "use that one with the toxic spit".