Balance is not usually an issue. If you really want to check, mark one blade of the fan with a marker pen and (with the the power OFF), flick the fan and see where it comes to rest. Do this a few times, it should stop at random. The mark should show you where. If it stops in the same position each time there is a balance problem. This will EVENTUALLY lead to premature bearing wear in the fan motor. However, the fan is not usually a separate spare part and is normally supplied with a new fan motor.
SO……… Don’t worry about it! Just make sure the fan blades clear the back panel and you’ll be fine.
Regards,
Penguin45.