Just pick the one that looks like it has the flavors you enjoy, then that’s usually it, unless there is an option to pick sugar / ice amounts. In Berlin they also let you pick what “bubbles” you wanted : you had the classic boba with some flavour inside them, but also chunks of jelly, or what I guess was tapioca pearls (more chunky, no juice inside).

Or you do like I always did in China : point to the one that looks best and hope for the best :)