I usually look at recent scans and then purge whatever hasn't been alerted much. Only one Toxic alerted? I note the name and purge Toxics. No Smalls? I purge Smalls. Not perfect, but it works fairly well. I also try and purge all of a size class so I know that any I see later are alerts. Right now I've purged all my Very Tiny and Tiny planets, so I know any new ones I see aren't to be purged.