Does anyone know about monitoring a manuscript for its relevance to the target audience? I can't find anything about it!

Having been writing for over 20 years myself, I'm sorry to say there is no 'target audience' for manuscripts. At least not as you posed the question. Your first critic is your literary agent or the publisher you send your manuscript to. If you are rejected once, don't give up, even the greats of today like Stephen King had his work rejected before someone saw his true talent. Keep the dream alive. And good luck.

Try www.writersdigest.com