If you cleaned the parts well, fluxed them properly and then soldered them by melting the solder with the hot part of the pipe or fitting and not with the flame from the torch, you should be okay. You could try pressurizing the pipe and watch a pressure gauge over a few days. If it was me, I would turn the water on and wait 4 or 5 days and see if there are any leaks.

If you really mean a union not a coupling I am not sure that it would be a good idea to bury it in the wall.

oops...I meant a coupling. Two days and still no leaks.

So, are we up to 4 days without a leak? Don't feel bad about worrying. When I do plumbing in empty apartments, I frequently turn the water on while I'm there and turn it off at night when I leave. I've often done this for 2 or 3 days. Never really had a problem, but I know the damage it will cause if something does leak and no one is around to see it. Better safe than sorry.

Still no leak. Am I alone? I'm out shopping and I'm wondering if my pipes are leaking. I'm having dinner with friends and I'm thinking about the pipe. Before bed I check the pipe...I cautiously approach the pipe with my heart beating faster until I am assured there is no leak. How do you pros handle the pressure?