Normally, such a confession wouldn't be based on memory. Rather, we expect to see this kind of configuration confusion come from some poor soul who purchased a used RV and the previous owner had the memory issues. But be that as it may, it could actually become an issue.
The answer to figuring out this problem has a couple of variables. If your dump valves are basically "uncovered" so you can see the pipes that go into them, the problem is often simply resolved. The black water line is typically large – three-inch size pipe – while the gray water is commonly two-inch, or even inch-and-a-half size pipe feeding out to the dump valve.
OK, but what if – what if the pipes are the same size, or what if those pipes aren't visible? If you can't easily remove the obstructing cover to get a good look at the supply line, or if they truly are both the same size, then life gets a bit trickier. Can you see the holding tanks themselves? It's likely that the pipe runs out to the dump valves will be more-or-less direct. That is, if the black tank is more rear-ward than the gray, it's likely the most rear-ward dump valve will be black water. But how do you know which tank is which? Ah, students, equip yourself with a chuck of 2" x 2" lumber, three feet long. Shut off the pump and the city water supply, step on the lever as if you were evacuating your toilet, and GENTLY push that 2 x 2 down the throat of the toilet, while holding the valve open. Now gently tap tap tap the bottom of the holding tank while your able assistant under the rig is listening for the tell-tale sound of tapping.
Finally, here's the "when all else fails" answer. Get thee to the RV supply store and purchase a clear, dump elbow. This clever attachment hooks onto the outflow of your rig's drain system, and acts as a go-between the rig and your sewer hose. At the dump station, put on the dump elbow, attach the sewer hose, properly insert the dump hose into the dump station port, and prepare to quickly pull (and be ready to shove back) either one of the dump levers. If you see that awful brownish liquid in the elbow, all well and fine, complete dumping. You've found the lever for black water. On the other hand, if you see gray looking drool, quickly slap the lever back – you've found the gray water handle. Dump the other one first, then go back and dump the gray.
Once you've properly ascertained which lever does which job, you'll want to code them in some way to aid your memory. Got a label-maker? Equip it with plastic tape and print out a label for each tank. Or use electrical tape as a tag for the black water, and maybe duct tape as a tag for the gray. Or draw a little diagram and post it in a compartment close to the dump levers.
Now if I could just remember where I put the keys to that compartment . . .