Basically, when it goes off it'll expand, obvious with a carton, less so with a tin but look for the flat ends bulging. Also when you open it, if off, it'll stink to high heaven.
Mostly probably OK to use to get messy. Flavour may not be as good once past Best Before date. Note Best Before and Use By are different things, Best Before means probably still fine after that date but quality may decline, flavour may be impaired, etc - but still safe. Use By means "will most likely poison you, possibly kill you, if consumed after this date."
If it's canned, it doesn't go off. Unless the integrity of can is lost somewhere. They found some of Scott of the Antarctics rations at a way station a few years back, and the intact tins still contained edible stuff.
Cartons, I don't know about. I'd be suspicious that DM1 is right and if it's an issue it should be obvious between the cartons expanding and any smell.
Love you, too
