You validate with the marketplace all those User Personas or Buyer Personas that you create. There is a lot of science to them backed up by hard data when it is required. In fact, they are one of the hottest design topics around and one of the FIRST things we create. Sometimes right after or right before that customer journey map we create. 
I have always challenged people in workshops to create their own persona and their own journey of incoming and outgoing workflows. It is a challenge for most to get very far removed from themselves. At the furthest point, I then ask them to create a persona of that person. If that person happens to be in the same room, we compare.
The idea of a persona is where we get the icon for drama, the two masks of comedy and tragedy. In theatre, the older mask were leather and shaped into magnificent forms, but on the inside the masks conform to the actor’s face. So on the inside, you have the actors face and on the outside the shape of the character.
Our personas are commonly created the same way. They conform to our inside thinking. We will have a workshop placing post-it-notes on the wall creating these personas. We come to an agreement and start with the next exercise, often never returning to validate the information. This exercise is a brainstorming exercise, not a user persona.
Others will create the material with some data they have collected and validate the information through customer surveys or interviews and other quantitative or qualitative data. These personas are more accurate but still flawed. It is hard to separate yourself from your mask.
Your perspective creates the way you see the world. When you are creating a persona you are taking events and people opinions that are heavily influenced by your culture and surroundings. Can you ever be objective with a persona? How many times when you have uncovered something uncomfortable is it justified as an exception?
That persona that you created about a co-worker above, how close was it to the actual truth? The truth is what you imagine for others is very different from actual experience.
Creating personas is a tough exercise, a few tips:
- Go to the actual place that value is created, the point of use.
- Observing, and/or interviewing with multiple people from multiple disciplines.
- Have people record audio and/or video of what they saw.
- Create stories about the interaction
- Have real users/customers critique your persona
- Use outside sources to collect part of the data and perform any of the steps above.
- Induce failure of the product/service and observe reaction
- Believe what others say.
This list is not meant to be exhaustive by any means. What other ideas are there for creating good personas? Creating separation from the internal mask?