Originally Posted by SquidPuppet

This might be a total waste of time and effort. The amount of pressure that you can put into a bare tube is too low in many cases to cause the hole to leak.

let me explain it another way. Your tire was at 50 psi and leaked down to 20 psi and stabilized there because 19 psi isn't enough pressure to push air out of the hole. It's even worse when the tube is out of the tire because you are only putting in a couple of psi.

If you want to test tubes before using them I'd recommend submersing them in water as your best chance to spot a defect.