What Orius said above. The townhall keeps a log of all events occurring over the past 5 days. One day just aide everyone directly from that log, then scroll thru your friends list to see the slackers.
Please don't ever send a spam message to all in your friends list asking them to leave that thread, with those who don't leave subsequently dropped from your list. Many players are doing this in an erroneous attempt at determining who's active. But all it shows is who responded to that message, not who has physically aided your city. And, many players find such threads annoying as well as pointless (some will even drop you from their friends list for doing it).