I check my cartos by pulling the drip tip off and taking a big whiff. If it smells burnt in any way, the carto is dead or dying. I usually run 5ml through a carto before there is a noticeable drop in vapor and flavor, or the flavor starts to turn. I have made it a habit off pulling cartos apart before they hit the trash, and 9 times out of 10, the wick tube is sooty, the polyfill is scorched, and the wick is encased in what looks like charcoal dust.