Peter Gibbons said:
See DMG, pp. 25-26.
The DMG says it should be pretty specific. Personally, I favor much more flexible Ready actions; it enhances realism.
"To do so, specify the action you will take and the conditions under which you will take it. Then, any time before your next action, you may take the readied action in response to that condition. The action occurs just before the action that triggers it. If the triggered action is part of another character’s activities, you interrupt the other character. Assuming he is still capable of doing so, he continues his actions once you complete your readied action."
I let there be a little bit of flexibility in the conditions:
"if any foe approaches within 5 feet, I attack with my sword"
You do not have to specify which foe.
If the conditions are questionable or vague, I allow for Spot, Listen or other types of rolls:
"If any foe comes through the door, I shoot my arrow at it"
If an ally comes through the door, on this one, if you make your Spot roll, you do not shoot. If you miss your Spot roll, you did not recognize him quickly enough to not shoot at him.
"If any foe casts a spell, I shoot my arrow at it"
If an enemy starts casting a spell without somatic components fairly far away (30+ feet), if you make your Listen roll, you shoot. If you miss your Listen roll, you did not notice him casting in order to shoot at him.
But, it is not automatic. You do not always notice everything around you.
With regard to the action, those I require as specific. You must state which spell you are casting, you must specify that you are tripping as opposed to doing a normal attack with your Spiked Chain, your must specify how much you are Power Attacking, etc.