To test for spoke tension, just hold a screwdriver lightly against the spokes as you spin the wheel, you should hear a high pitched 'ting'; if you hear a dull sound more like a 'thud', you have a loose spoke.
When most cycle shops replace a spoke they do not re tension the whole wheel, they only bring it into true, that is why you had been breaking so many.
As a heavyweight myself, been over 15st for far too long, I can ride on 32 spoke 3x wheels with 23mm tyres without any problems.