If you can see them - The black tank will be a 3" valve. The grey and galley will usually be smaller. I'm sure there are exceptions though. Also, your toilet will be directly over the black tank.
If you can't see anything, and don't want to open any coverings, running some water will tell you quickly enough.
Oh, and get yourself a clear plastic elbow to connect to the dump outlet. Then you can see the flow and it will be pretty easy to tell which is black waste.
It is amazing to me that they would be too cheap to apply some labels. Sheesh!