Reverse is in and out on my transmission

What is your question?

I have poured too much money in a now very nice 325ic. If i'm going backwards up hill the reverse slips. All forward gears work great. I was told it might be low fluid and that this is very rare in the 325i's. Since there is know why too check the fluid levels without spending a ton of money, does anyone know or have a suggestion? I know the worst case from a rebuilt one to a new one.(transmission)When the car is on a level road reverse works....it's only going backwards up hill??? Thank you in advance!