Kinda feel like I am telling my little secret here but lets hope only cardschat players see that.
This tell works wonders for me 90% of the time and its a combination of online timing and betting tell. When you are heads up and your opponent check the flop in a normal set of time, and then at the turn he takes a whole lot of time to just check the turn then you should bet the turn with whatever your cards are and you more likely to see a fold than if he checked back at normal time or checked quick.
I believe thats because he either hit something weak and takes time to think if it is worth betting and since he didnt bet, he hope you dont bet and ready to fold if you bet. Or either he didnt hit anything and trying to project that he is thinking of betting or setting a trap so he can see cheaply a river and if you check he intent to bluff the river. Or he just take the time to think if its worth bluffing and concludes that is not.
You have to take account that I am talking about low stakes here, I dont know if this holds true for higher stakes that there are bets in every street.