Actually, it's already implemented, native and automatic :).
If you use RemoteID, SQLIContent::create() will automatically check if a content already exists with this RemoteID. If this is the case, it will be returned. Then, when you'll publish it, the publisher will go through all the attributes and check if there has been a modification. If this is the case, a new version is created, if not the content is not modified.
You can check the doc in the "Simplified content API" part : http://svn.projects.ez.no/sqliimport/trunk/doc/README.txt
