If this is from an AD, still have them open the back and check it out, then have them do the pressure test for a Rolex diver's watch (it's the same contraption for any diver's watch, btw, Omega, Doxa, etc.). If it's by private party, go to a Rolex AD, etc. If it's over the internet, don't send them a check or wire them money, go with credit card, but that's taking a big chance unless you're familiar with the seller and have done business with them before. Caveat emptor!
Looks nice, just hope it's not a Chinese ETA-brass "movement" inside.
=:^D