In the case of the two M4's bottom one is real and top is airsoft and also a GBBR as Kaos suggests. The forward assist is worn because it's got a crap paint job from factory
The large grip on the bottom one is a magpul grip whereas the GBBR has a stock grip.
I'd argue it's very hard to tell unless you're experienced with the particular firearm. But even then there would be no way of being sure.
The last two Sig's Styrak posted up I would have had no idea which was real and which wasn't had he not have told us. Maybe if I owned one of each (airsoft and real Sig) I'd have a chance of telling?