I am not sure how accurate this is but for a general rule of thumb water proof testing I make sure everything is greased and ready and then I fill a body of hot water. I then place the watch in the water. The increase in temperature increases the volume of the gas inside, which will cause it to leak from wherever there is a problem, if it starts to leak, then you take the watch out of the water (well before it stops bubbling, otherwise bye bye watch). This has worked for me but of course I have no idea of the general pressure that this generates.
Of course be very careful if you try this and don't blame me if you lose your watch. If it was anything that I was that petrified of losing, I'd just get it done for real.