You can usually tell as they're very vague. Oh it's his leg.... yeah which part? How?
Illness raises flags too bit usually is actuaally an illness
That and you hear rumours. having said that some people will randomly make stuff up like how a prem footballer had smuggled ketamine onto a commercial plane and was handing it out to everyone
and he had a picture with said footballer and it was like really????? You really expect us to believe said footballer gave you drugs and a selfie??? He admitted way afterwards he was joking I think but damage done.