Just like above I use a multimeter. Test the ohms and if it reads at all, it works. I never have duds EVER, whenever someone says that they ordered cartos and 2 of 5 were duds, I think they didn't test different things to make it work. Pull the posts, use extenders, etc. and you can almost always make cartos work. I bought my multimeter for 16$ at walmart (overpriced) and it's kind of a weird one, have to set the ranges and all. PM me if you buy one and don't know how to use it. Sorry if it's rambling, hope that helps.