Paul never met these 500 people. He says he is relating "which I also received," which means he was supposedly told about them, presumably by Peter.
And, in any case, he is writing to people in Corinth. How could they check out the story?
Unlikely that he hadn't met them. He spent a lot of time in Jerusalem, and he was writing his letters only 17 after Jesus' death and resurrection. People who had been Jesus' age - 33 - at that time, would be only 50 when Paul wrote.