So if I get a payment for 100 bitcoins composed of 20 smaller balances from 20 different accounts, and I trace those coins back and find that 1 week ago they all belonged to a single account, it's likely that the coins were passed through a "send to self" mix...
Yes, after posting I realized that self-mixing creates a mostly-self-connected sub-graph. You really need to start with multiple transactions in, ideally randomly occuring during the mixing process, and not spend all your bitcoins at once.
Somebody who knows a lot more about graph theory, statistics, and probability than I do should chime in and tell me how I'm wrong or come up with a good metric for the ideal amount of mixing (I bet too much is just as bad as too little). Analyzing the connectedness of the existing bitcoin transaction graph might be a good place to start.