It needs to be done with air-delivering gear underwater, and that can be done at the surface. All it takes is a diver who can lie on his or her back.
In fact, I can think of a specific dive which was saved by doing the bubble check at the surface. Team of 4. One diver had brand new tanks. He went on his back and it was noted that dime sized bubbles were streaming out of his right post. So we re-threaded his first stage and checked again. Still leaking dime sized bubbles. At that point, one team member took off a dry glove, completely disconnected the first stage (leaking diver holding himself far enough out of the water with my help and that of the anchor line), and found brass shavings (from the machining process) on the o-ring. They were preventing a proper seal. He brushed them off, did the same inside the valve, re-attached the reg, and upon checking, the problem was solved.
This was a training dive planned to 120' for 15 minutes with a lot of skills to be performed on ascent. It would have been impossible to properly/safely complete with one diver unable to donate.
So what would have happened if we'd waited to submerge for the check? Well, we'd have seen the bubbles. Leaking diver would have shut down the right post and purged his reg. Bubbles would have stopped. Turning it back on, bubbles would have started. Back off, purge, team member tries to re-seat the reg. Back on, bubbles start. Conclusion? Non-fixable, he can't donate (with a team of four, we'd have been okay, but it certainly would have made those s-drills a bear): call the dive.