If you gently test the lid with your fingers and the lid comes off with little effort, it's been compromised...If you try twisting the lid off a sealed round tin, it won't come off by hand unless you release the seal first...If it twists off by hand, it's not sealed. If the lid is convex rather than concave it may be compromised...If the tin is dented, it may be compromised. I think it's fairly obvious of a seal has failed on a tin. For me, I'm simply not storing tobacco in the flat tins anymore. It'll age fine without the tin. End of story. I've popped (almost) all my flat tins and found a couple of them to have no seal and containing dried out tobacco.
The seals on these flat tins are permeable, which means some aroma molecules and gasses can seep out without decreasing the internal pressure. I think off-gassing would increase the internal pressure (?), but that process can't last forever. Nature abhors a vacuum. Eventually the pressure inside any tin with a permeable seal is going to equalize with the atmospheric pressure. Then your seal is potentially compromised. How long will that take? Years? Decades?...It would depend on several variables; meaning there's no way to predict it. Eventually though, without enough internal pressure to effectively suction the lid in place, moisture will start to equalize with atmospheric humidity. If you're in a dry environment your tobacco will eventually lose most of its moisture. How quickly that happens would again depend on certain variables. Of course, as you've stated above, dry tobacco can potentially be salvaged and smoked, but a dried out tin is not an ideal storage vessel for finely aging your most cherished blends. YMMV.
