professional reader/critic

a bit more detail would almost certainly be helpful

Make lists of all the similarities between the books and all the differences. That should help you make a more measured judgment.

If you're willing to spend a substantial sum of money, you could look for a pro independent editor. Never used one myself -- Elizabeth Lyon is the only name I can think of off-hand. Definitely do serious background checks on any possible reader/editors -- there are scammers aplenty to avoid.

If you decide you're different enough, get the proposal out there and let agents tell you how you're doing. They'll bite or reject for free.

Sharpshooter, if I were you I'd just list what you need in the "beta readers" section. Why not have other writers do it for free?

To be honest, a proposal doesn't take all that long to complete. Couple of sections, a couple of sample chapters, maybe a weeks work at most if all the considerations, research, etc. have been done - I'm talking just to write it out.

An easy way to tell would be to send it to a VERY small batch of agents, preferrably not high on your wishlist. If you get any feedback that mentions that, or no feedback at all (presuming your work is actually sought-after,) It may be a good litmus test. You don't burn any options you were dying to use, and you never know what may happen.

I know you're not supposed to send out stuff you don't completely believe in, but I'm guilty of doing this before. I'd just run with it, and save yourself the money of hiring a professional reader. I don't know how much somebody actually qualified and professional enough to do this would cost, or if you could actually even find them to be honest.

IDGS, I think that is a good idea. What you're not sure of and can't find out, try it first with a small number of agents/editors first and see what happens. Makes sense.