depending on the mix, the pressures inside these cans are not very high, especially in the case of Butane. They are quite sensitive to temperature though.
so if they have already survived a heat wave or two they are probably not just about to burst of their own accord. The main danger from them is not that they will burst explosively, it is more that the contents are very flammable. By corrosion they are most likely to fail to a leak rather than a rupture, and a rupture might be dangerous if there is a source of ignition nearby; there are many kJ of energy within each can. But then there is inside an old can of WD40 too and most folk don't worry about those.
If they are the sort of canisters that are pierced to open then you shouldn't worry overly about piercing them; this part of the canister is very much overengineered against splitting. The main danger when using them normally is that the seal doesn't seal properly at the top of the canister, in which case you can have a leak that can result in a second flame; this might still be the major worry still; the rubber seal may not be effective on a slightly corroded canister surface.
If you want to examine them under conditions of improved safety, bung 'em in the freezer for an hour or two first; the internal pressure will be much reduced. In fact in cold weather butane canisters will usually dribble liquid butane out, so cans from the deep freeze can be pierced and left to slowly disgorge their contents as they warm up.
If you want to pressure-test the canisters, you could (outside) drop them into a bucket of hot water and then (swiftly) retire to a safe distance. If they don't rupture under that treatment then I'd assume they are still fairly safe to use in a normal way.
cheers