I ditto the brilliant advice that Housil, elandil, parafireboy, Ruleryak, and ratattoo all say.
I just do the visual/smell/taste. Taste the questionable food on your tongue, if it stings, burns, or feels sharp in any way - spit it out!
If a pack is bulging, or damaged, skip it! I had German EPA II bierwurst that had a damaged outer lid, and ate it. I really shouldn't have, but I was still fine. The gelatin made a seal around the puncture - it would have gone bad within a week-month from then, and I suspect it happened in shipping.
Elandil is right, have had bad dry milk in breakfast muesli - I also got really sick from a 1977 Chicken Stew last year. Bad call and it was a tough one - the vacuum seal hissed too and it all looked fine. I ruled that as a defective product to begin with OR bad dry milk product in it. I will never truly know.
Be careful with the vintage MRE nut cakes - the nuts go rancid most the time. If the nuts look dark in color - and if the thing has a slightly fruity smell. Stay away. Taste a small bite, if it swells your tongue, spit it out and wash up and brush your teeth.
It takes 2-5 minutes for your mucous membrane to pic up rancidity from nuts.
Pinholes, bulging packs, tears. Any of those going on - pass on it.
Smells off in any way (sour, stinky, rancid, foul, metallic, bleachy, sharp) - pass on it.
Tastes bad in any way (tingly, swelling of mouth/tongue, sharp, coating your mouth with an unpleasant grease, burns, bitter, metallic, or chemical-like) - pass on it.
I have eaten food from 1950 and lived to tell the tale. Follow these methods of your perception & senses.. and you shall be legendary with your buddies.
Stay away from old dry milk and cheese too. Not worth it!
Also, do NOT non-nonchalantly eat Ukrainian Rations. I had some with a date of production of September 2014 - the cans were bulging and for some stupid reason, I thought "heck, maybe they were just overfilled cans" and ate 'em anyways. They were spoiled.
Folks, I had been sick for a good time of 2015 and didn't tell anyone - it could have been an exterior issue from Rations, and I ponder the various possibilities of what those possibilities were, but it ALSO could have been this or the 1977 Chicken Stew...I had E. Coli poisoning... sweating all the time.. and among other symptoms. (*Edit*) (and yes Housil, in Key West i was sick.. but I knew it was not contagious. Had bladder issues, and was sweating bullets!)
Beginning of October, i fell very ill and almost died.. kidneys, bladder, everything really.. super infected - I have a massive medical bill from it all too. (someone with no health insurance like me should be careful eating old Rations)
Was it from Rations? From other bad food? Something else? I will never know..!