Hello, The correct preposition for longer periods of time, such as months, seasons, years, is IN, therefore "Which season are you in?" is the right choice.
ON is used for days and dates, e.g., I`ll be home on Sunday I`ll be home on the seventh of October.
Oh. Well, to be honest, I`d just go with the simple, less confusing "which season are you watching now? " form; but if I were to use a preposition instead, I`d intuitively go with "at" ( not sure how correct that would be though. ) To describe an event from a particular season , I`d use "in" : Rachel marries