Luca: For me the key is "one motion" (2011 TDA Rules # 36)... are those chips pouring out of his hand non-stop or "dribbling" one-sey, two-sey. If you catch me in a good mood I'll tolerate the first, but the second not a chance ever. Slightest hesitancy and the bet is over, and if I'm not in a good mood I'll rule the first a min-bet / raise also, I just don't like that kind of betting.
It used to be more common to see a player "shake" his chips into the pot, as long as the shaking continued we tolerated the continuous bet... I think alot of TDs are getting away from that now and just insisting on "one motion", and that's all to the good as far as I'm concerned.
I will channel the late Al Wise here "if it looks like a string bet, it is a string bet", ultimately that's what you have to go with. If you rule other than what the player intended, i.e. you stopped the bet when the player intended to put more out, it's his fault, not yours. "It's players responsibility to make his intentions clear" (this language will now be in Rule 2 of the TDA Rules, btw).
Of course this is all different if the table uses a betting line or forward motion, in which case all the chips in the player's hand are probably going to be ruled bet if he extends his hand forward.
Thanks alot for the great question.