2 Obvious choices are:

  • People who are a good choice for the book’s target market. They should ideally be avid readers of books in the genre or category in which your manuscript lies.

  • People who have some writing/publishing experience or understanding. Beta readers who are fellow authors or editors are also the best and have the most accurate, informative feedback.

Active forums of Beta Readers:

Beta Readers & Critiques

Critique Circle

Writing.com

Goodreads Beta Readers Group

Beta readers and critique partners

Absolute Write