I like this one :

When a player is yet to act and looks at his chips, he's hit the flop and is thinking about a bet. Most people will do this subconciously.

or :

when the flop lands three to a colour, and a player takes another look at his hand, he didn't make the flush (although he may have one card).

[img]/images/graemlins/wink.gif[/img]

I think the obvious one are the most reliable.