I feel like another good tactic, if you know multiple languages, is to switch between them. If they don't get confused and/or as to why you switched, and/or just switch with zero hesitation, that could be a potential sign.
Probably works best if you know like 3 languages and at least one is something completely detached from the other 2. I mean, what are the odds you run into someone else who knows, for this completely hypothetical scenario, English, German, and Tagalog? Or another 3rd language far removed from the other 2?
Edit:
I feel like this would work best on a first "meeting" kind of deal. You get a DM or SMS or call from someone and they don't even acknowledge the conversation has eventually shifted languages. You haven't given them any reason to believe you speak multiple languages nor any indication you speak a far removed language. But they don't acknowledge the shift and keep the conversation going at all costs. That kinda thing.