There are ways to find songs that are 'shared a lot'. Songs that have been bought from the iTunes store don't have any DRM, but they do have your iTunes account ID stored in the metadata for every song bought. So, if the copy you snagged happens to be the unadulterated copy (ie not converted) then it's going to be fairly obvious that if thousands of people have this copy then it's been acquired less than fairly.
Other ways is to probably have a checksum of every song sent back to the Apple servers for checking. This isn't 100% at all, but with all that info, it can be cross referenced against everyone's collection, and various other sources to possibly determine it's origins.
Then there are all those who never check for little comments from their 'acquired' songs.
None of those are surefire ways to determine your culpability, but if you have thousands of 'red flagged' checksums of songs, it's probably cause for concern.
One more thing though, Apple makes far more profit from selling you iDevices, and iTunes Match is designed for iDevices. If they arrest you, you'll probably never buy another one again.