I couldn't find a way to simply compare all the systems in SCCM to all of AD. I had to select specific OUs, and compare that to specific collections. That's not how we organize systems in SCCM. Historically, I do an export of all computers from SCCM into a .csv, then the same thing from AD, then put both lists into BeyondCompare, to see things that are in one place but not the other. I was hoping to do that with RCT, but it didn't work out that way.