You can typicaly find a sniffer at a pawn shop. Take a can of R-134 along with a tapping hose so you can release a bit of 134 and test it. I have found tiny leaks with mind, so I no longer use the dye. But then, it is a personal preference depending on which one works the best for you.

If you compressor seems locked-up it just might be the compressor clutch. BEWARE--- If your compressor clutch is frozen up then just replacing it will likely cure the trouble for only a few weeks. When the clutch is worn out the compressor is just about shot as well.