You can tell when they go bad (have been exposed to moisture), then end near the striker cap tends to swell and eventually burst. Once they start to swell, if you try to ignite them they just disintegrate.
I used to rotate mine through,, boat until expire -> wife's car -> my truck, then burn them (by that time they were falling apart). Whenever I got a new set, into the boat they went and start the rotate all over again.
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