Testing circuit breakers?
The only way to test a circuit breaker is to present it with various loads and see how quickly it pops, and measure whether that matches the formal specifications for that breaker. Theoretically, the folks selling refurbs SHOULD have done exactly that testing, and the manufacturers should have done so at least on samples from the production line... so this becomes a question of now much you are willing to trust your supplier..

Quick tip: I too have a breaker box that takes a format of breaker which is no longer easy to find. However, many houses in my town were (re)wired at about the same time and use the same breaker, so while the major electrical suppliers don't stock these the local hardware stores DO. You might want to check locally before assuming that refurbs are your only option. (My electrician knew this. My solar installer's electrician didn't, and wasted a few hours until someone clued him in.)