[dparm]

Just change it per BMW's requirements which you are incorrect about, by the way:

Brake fluid = every 2 years, regardless of mileage (mileage doesn't really wear down brake fluid, time does)
Rear differential fluid = every 3rd oil change (time doesn't affect this fluid much since it's a sealed unit, but mileage does)

I don't know the manual trans fluid interval, but it's probably every 2nd or every 3rd oil change. It will probably be listed in the owner's manual.

The only way to know what condition they're in is to pull a sample of the fluid and send it for analysis at about $35 per sample. You can certainly do this if you want to monitor the fluid over the long term and establish a baseline, but a single sample won't tell you a whole lot.

The dealer can check what the car needs, and what has already been done. I hope you already did this before buying the car, though. Anyway, there's no use wasting money by just guessing or arbitrarily replacing fluids that have lots of life left. Even if you don't want the dealer to do the services, they can at least give you the car's history.