It's happened a few times I'd be healing a guy and then wind up back stabbed. x_x Now I always shoot before healing which gets tedious. Is there any other way to tell?

Not really, you can heal someone with a medigun and then turn 180 degrees and face the other direction and the healing won't break, otherwise just keep strafing around like crazy and make the spy's job harder.

Are you using the Blutsauger? That doesn't work anymore. Disguised spies still take damage, but you don't get healed for it.

just be cautious of your team mate its usually hard to protect your target from spies especially heavies.

I find that if I'm healing a heavy, it's slowness makes it possible to circle around him, so he'll get backstabbed and you can kill the spy.