I'm not sure how privacy concerns, rights, etc. factor in, but for argument's sake, let's assume Tesla could use all the data available about us and our cars to determine "taxi / not a taxi." I don't think it would be that difficult. They'd start by looking at the cars that use the superchargers --A LOT--. Then look at the travel patterns for those cars. Are they going to places taxis go to a lot, like airports, train stations, and hotels? I imagine if they need to they could even look at trunk and frunk openings, (I've read that at least the frunk openings are logged--true or not, I don't know), weight changes in the cars (which might have to be inferred from changes in efficiency, etc.) If you're talking about a car driving around a city and making stops, it shouldn't be hard to tell if people are getting in and out constantly or not.
I think if Tesla's goal was to identify cars being used as taxis, it wouldn't be hard to do.