Good point, and one which has tempered my reactions in the past... I don't know of a reliable means to discern between filtered vs. private entries, which then impeaches the evidence, so to speak. In the past, I've looked at posting patterns -- someone that appears roughly five times per week, then persistently appears only twice/week -- adjusted for any known crises, travel, or changes in living conditions. And I usually attribute sudden spates of hidden entries by someone to private entries.

Otherwise, I have been known to skim entries by other people on a given person X's friends-list, looking for their let-slip references to topics discussed in filtered (vs. private) entries... when I have time, and when I'm trying to convince myself whether or not person X is filtering me consistently (and hence would probably rather see me withdraw or disappear).