I would look at the packaging before eating. If everything is still sealed airtight you should be good, however just like anything else if it looks or smells funny you might not want to eat it.
I open one up a few years ago that had been expired for about a year. Didn't notice any leak but when I opened the foil there was no mistake. Didn't need to smell either. That damn thing would have gagged a hound off a gut wagon. Black as hell as stunk to high heaven.
a swollen brown bag mre is a bad thing. Do not pop the bubble. I guess they don't use those anymore, but MRE's aren't great to begin with, I would imagine a rotten one would be bad.
When in doubt, throw it out. I personally would use them as exploding targets. A great revenge tool, poke a couple of holes in them, slide them under a hood, seat, a porch maybe a an odd heating vent. As they ripen let the fun begin.