Here are some things to take into consideration:
- Do they like your genre? If they don't like your genre, that could taint any feedback you get. They might hate your vampire book because they don't like vampires. They might like your battle scenes because they never read one before and don't know a good scene from a bad scene.
- Do they give you the type of feedback you want? You might be looking for everything from grammar, consistency to plot feedback, and they only spellcheck the document. The Beta Reader might only talk about what they thought of the ending.
- How fast is the turn around? If you need feedback within a few weeks, make sure your Beta Readers is on board with that expectation.
- Too nice or too tough. A lot of authors state, "Be honest. Tell me the truth." Every author has a limit to the thickness of their skin. Ask for specific feedback, and take it or leave it. If a Beta Reader is too critical without any specifics to back it up, he/she might not be for you.
- Related or Conflict of Interest: Your parent, sister, partner, or work colleague might not be the best person to Beta Read. Anonymous Beta Readers don't have to share a bed with you or a conference room.