How would I check thousands of entries to see if they're safe to remove?
Click the "Legend" button in the upper right corner of the screen when RFA is either running a scan or has just finished, then look at the bottom three entries in the legend window that appears. They explain what the icons mean, and the program automatically marks for deletion anything in the "Regular" (safe to remove) category, which usually includes the large majority of the entries found.
You can also see "Safety levels (levels of user experience)" in the Help file under "How to use Registry First Aid / Safety levels (levels of user experience)." Help is accessed from the upper right corner of the home screen.