In snooker I know the rule whereby a player cannot use a ball (off the table) to check whether a colour will spot or not after potting it. (the referee has to do it). However what about if the player is wondering whether to play a certain colour and lays his finger near the spot concerned to guess whether it would be a wise move. (assuming he knows the length of his own knuckles compared to the diameter of the ball)-would that be allowed?
And - a snooker ball is 2.07 inches in diameter. The balls contact each other just over an inch above the playing surface so measuring on the surface doesn't really help.
No it isn't. A colour must be nominated. It then becomes the ball "on" and if any other colour (or a red) is struck by the cueball first (or potted) it is a foul. In practice, especially in the professional game, it is not usually necessary to verbally nominate a colour unless there is doubt as to the player's intention (say, if two colours are close together).
I don't think there is anything in the rules that prevent a player from touching the surface of the table. In fact you often see players removing dust or small particles from the baize.
The rules do provide penalties for "unsporting conduct" which is define as:
...any conduct by a player which in the opinion of the referee is wilfully or persistently Unsporting; or
(iii) any other conduct by a player which otherwise
amounts to Unsporting Conduct;
But I cannot see how the action you describe would fit that bill.