One way is to press test with nitrogen, check with liquid leak det. solution. Another way is to use a good electronic leak detector while unit still has a refig charge or add nitrogen with a refrig. tracer. There are dyes you can put into systems, I found they are messy and hard to get out of your guages,
Both Superseal, the one with the one time use hose and the other one that you can reuse a hose that is sold seperatly. Had a lot of trouble with superseal. Could not get it out of the can and stopped using it. So far with the other [sorry I can't remember the brand but in the DC area I've only seen the two ] I have been back to add freon on one. I'm not inclined to use either anymore.