If I'm buying a used car, how do I know it wasn't used before as an Uber or Lyft car?

Check out this thread on this same topic here.

or check for barf stains in the back seat.

check to see how many miles are on it, and compare that to how old it is. you can usually tell if the car was used for a service like uber if there were a lot of miles in a short amount of time

I think if you ask them, they will tell you? i'm not positive though.