Different Cons have different regulations for cosplay weapons, so it's always worth while keeping an eye on those.
Weapon checks are part of security going into most Conventions these days as well, they quite often will fix something like an orange zip tie on a trigger guard or barrel once they've checked it. Similarly for cosplay "blades" or other things.
Bows tend to require unstringed as a thing.
Quite a lot of prop guns will come with a bright red / orange cap that can click onto the end of the barrel so that's another 'obvious' visual sign it's not real.
Funny thing about the original Star Wars prop guns was that due to budget, they had to hire the WW2 guns they used as a basic form. Then they could only do certain changes that could be removed afterwards.