Since you can't put a foreign substance on your club....

during a round like impact tape, or Dr. Scholl's foot powder to see where you're making contact with your driver, I've heard that marking your ball with a dry marker will transfer that mark to the face of the club so you can see what's going on. You can wipe the mark off your club immediately.

is that legal?

Well you can mark a ball for identification. You're not directly applying anything to the clubface to influence the flight of the ball. So I guess it's legal. Hey, I guess you can keep track of how well you're making contact with your driver throughout a round.

I can almost always see a mark on the face of my driver after a shot without putting anything on it but I believe that's because the face of my driver is black so it shows marks easily.

I draw lines around my golf balls for alignment and identification purposes using a permanent marker.  At the tee I will line up the ball so that the drawn line is vertical on the tee.  Even though it's a permanent marker the ink will transfer onto the club face and provide me with feedback on where I hit it on the club face.  This only works for a few times per ball but that is good enough for me.

I'm sure dry erase markers would work even better, but make sure you use a color that will show up on your driver.

I have a regular silver-colored metal face on my driver and, even after 2+ years, as long as it was clean before the shot, I can tell clearly where the ball hit the face. I also draw a little green dot on my balls and if it happens to be aligned with the face on a shot, it's clearly imprinted on the face after the shot.

Foreign substance? Another reason to buy American.

Yeah, I have no problem seeing where the last impact was on my driver.  Just make sure it's clean before each strike.