what is the thought process I should be adopting in deciding whether this is a value betting spot or not?
look at the action preflop, and put him on a range of hands that can call preflop. Make sure you do this honestly, rather then doing it to justify a river bet. base this on experience with him, or of experience with reasonably competent players. Obviously the stronger the read, the more accurate your assesment will be, and thus your actions in the hand will be clearer. Then ask yourself of these hands, how many of them will call a river bet with a hand that I can beat. if the answer is not many, but they figure to have a pretty wide range, then letting them try and steal it on the river/inducing a move by the fabled weak lead is preferable. Its not just a case of how much you are willing to commit to the hand, but a case of how, and calling in these spots gets you a better return on your money then being the bettor, for obvious reasons.