First of all, let me start by saying that I think every writer should have critique partners or beta readers who read stuff before it is sent out the the wolves. And they should be people who will tear the writing apart. A good CP should simultaneously point out everything awful in your work while also telling you how awesome you and your writing are.
I’ve been lucky in the sense that I’ve had a pretty easy time finding great betas. BYU has a great writing community, and as an editing minor I was able to meet a lot of people in my classes. However, I’ve met great beta readers other places too. Writing conferences or other similar events can be great. I’ve even met several of my favorite betas on Twitter.
Anyway, I know a lot of people who have betas read their manuscript. Your query letter should get the same treatment, if not better treatment. Especially since it’s easier to find CPs for a query letter (it’s not as much of a time commitment). I recently made my writing group look at one of my query letters four times. I’m pretty sure they’re sick of reading it, but it helped me a lot.
I recommend two types of CP for your query letter. At least one who has read your manuscript, and one who as not read the manuscript at all.
The person who has read it will be able to say “this is an important part of the plot, it should be in the query letter.” The person who hasn’t read the manuscript might notice things that make sense to you but don’t make sense to somebody who hasn’t read the story. Both types are helpful.
So do yourself a favor and find some good CPs for both your manuscript and your query letter. Your work deserves it.