^^^ What he said
I test almost all my developers now by doing the leader dunk before throwing a film into them. Just as Mike does, I immerse the leader in a little dish of stock or working strength developer (depending on the brew) and leave it there for five to ten minutes, as appropriate. If it doesn't turn black I don't use the stuff. This has become so routine I don't even think about it now. Takes no time at all -- it sits in there while I fuss around prepping all the other bits and pieces for the developing session. I do this with fixers too.