A persona is not intended to be an accurate representation of your customers, but rather a sort of average representation of a typical class / type of customer. That makes it very difficult to validate whether your personas are good choices.
Additionally personas are chosen to try and represent a wide range of who you think your typical customers will be. So it's almost certain that at least one of them will not match up to people that you have done UX testing with. That makes changing your personas based on empirical evidence hard, as any one person is likely represented in some way by parts of two or more personas.
One way that you should improve / validate them is when it is clear that you have not considered a persona that is being shown by your UX testing. Add a new persona then.
What some frown on, but I often find useful, is to make a persona a real person. Someone that I know that I can do UX testing with and who represents my target audience. There aren't many people that are typical enough to do this, but when there is, it makes it more real. That said, I would never have all of my personas correspond to real people, as real people are not enough of a stereotype.