I don;t know the specifications the military had for 550 paracord, but undoubtedly it involves type of nylon for the strands and sleeve, color, test strength of the sleeve and strands, type of braiding in the sleeves, whether or not there are any splices in the inner strands, maybe some temperature testing, etc, etc.

It's hard to know if a company advertising their 550 paracord as mil-spec is "mil-spec" or not. Who's going to test it? FBI? Army CID? Consumers? They're purveyors after all. Some other company is making the paracord and they are the ones claiming it's mil-spec.

Anyway, I look for it when I buy paracord. I just bought 400' a while back, of varying colors. It was all advertised as "mil-spec" so I assume it is, but there's no way that I know of to be sure. Certainly, if they're selling it to the military then it's going to be genuine. Then the unused (same stuff they would've sold to the military) is probably sold out to retailers.