there's other advice on this, but my method is: visit everyone who's on any of your lists - gave you a gift, left a tip or responded to a request in your news feed, or wrote on your message board in the past couple of days (also visit stormie). whether you tip or gift them during your visit is irrelevant, just by visiting, they will have dropped to the bottom of your neighbors list in the social tab. so now everyone at the top of that list (above stormie, as you visited him too) has not contacted you recently.
some of the non-contacts you may want to keep because you know them, but otherwise, i've tried a couple different things - sometimes i go check for rotting food (and usually make a mental note of which ones have it and check again the next day - they may just have gotten caught up and missed their serve times), sometimes i tip & gift and see if that gets a response, sometimes i leave a message flat out asking if they're still playing the game. i usually don't delete without really trying to make sure they've actually gone, but that's just me.
and yeah, it's been suggested and seconded and we'd all love it. i'm not holding my breath.