How do you know when to trust someone or dismiss it as just small talk or BS?

A few years ago a local radio talk show DJ had a 90’s rock star,in the show he co-hosts.He mentioned:he played the bass guitar,the rock star told him in 2 weeks,he would be playing at the House of Blues and the DJ could come over,he would bring him on stage and they could jam. The DJ took time off to practice, bought a new bass guitar,after endless preparations,,he showed up at the House of Blues, the day of the concert just to be turned away at the door as just an idiot not on the list and no one knowing anything about any special arrangements made for him to play with the band ! If you would have been in the same situation would you have taken the rock star seriously or just dismissed it as BS.
I have known other individuals who have been in similar situations and it turned out the person making the offer was not serious or just forgot a few minutes later.
How can a person protect themselves from being made a fool ?