Beta reader–someone who gives you feedback on your book.
Beta readers and critique partners are an integral part of almost any writer’s process. They are the people who will give you honest feedback on your book and even help with revisions.
Where, if anywhere, during the process of querying and revising, do you see value of asking beta readers to read and comment on manuscripts? What are the pros and cons of engaging beta readers before querying agents, and what should one look out for?–From Questions for the Blog
As early as possible.
If you plan to use beta readers I would most certainly bring them in before submitting to an agent. Remember, this is the stage where you want your book to be complete and as perfect as possible. If you believe a beta reader is part of that process bring them in before an agent sees the book.
The pros of a beta reader are having someone who has the distance to see things you might not see in your book.
The cons are of course having the wrong beta reader who has a vision different from yours for the book.
Overall though, getting someone you trust who is willing to give honest feedback is always invaluable.