WCrawford said:
PHB p 160 under 'Readying an Action': "You can ready a standard action, a move action, or a free action. To do so, specify the action you will take and the conditions you will take it."
So you must specify that you are readying an action to counter spell and what conditions will trigger the counter spelling attempt.
Well, one could argue that the only thing that you'd have to specify is whether you take a standard action, a move action, or a free action.
I still think that you'd have the choice after readying for someone to cast a spell that you'd either Distract him with or try to Counterspell.
But I think that since you have readied a standard action that you'd would be unable to identify the spell being cast without choosing the option to counterspell, which would mean that you can't go back after you've decided to try to identify the spell.
Just my interpretation...
Another question is whether a targeted spell would fail if you readied a move action (a move action that actually involves leaving you square) if you'd trigger the move action at the point the enemy caster finished his spell.
You'd have no idea what kind of spell the enemy caster is throwing, but the question is whether targeted spells fail if you moved out of range or behind a wall, or even if spells that require a ranged touch attack hit you at all, even if you moved only one square? This might be a handy way to avoid a ray of Disintegration or even the Scorching Ray spell.
And even if you don't agree that you only have to specify whether you take a standard, move, or free action, you could specify you're move action like just when he finishes casting his spells I will move 20 feet behind the next wall.
-Malcer