If you move all your songs that you are about to delete to a different directory, iTunes should no longer be able to find them, and you should be able to hear the matched version and inspect its encoding. If you regret it, it should just be to move the songs back to where they were.
Haven't tried it myself to test it with one or two songs first!
I already answered a related question. I may not know the most definite answer, But I read that iTunes Match uses Gracenote's MusicID program. It seems MusicID uses the following techniques to identify songs.
MusicID-featured Identification Technologies:
CD Identification: Using the CD’s unique Table of Content (TOC), MusicID can identify CDs released around the world. Even with multiple pressings and release dates, patented fuzzy matching enables accurate recognition.
Stream Identification: Using an audio waveform fingerprint technology, MusicID can identify music with short audio samples anywhere in the song.
Digital File Identification: MusicID can identify individual songs or albums in a user’s collection regardless of source, format, or tag information. Utilizing a multi-step process which combines stream identification with textual information analysis from the file’s embedded tags, file name, directory path, and other related files in the user’s collection, individual or groups of songs can be matched to albums.
More here.
To answer your question, they perform some fuzzy text matching algorithm to map meta data and also perform audio waveform analysis.
Best Answer
I don't think you can do this with a single library, iTunes match will check everything in your library, under the 25,000 track limit, to try and match.
One possible alternative would be a separate iTunes library for the files you don't want to match.
This separate library could also contain the actual files that have been matched by the other library as well but not be enabled for iTunes match itself.
You could then use this 'super' library as the main one running the iTunes match library occasionally just to keep iTunes match up to date.
How to run multiple libraries