There are my own opinions, with which I agree:
Barcode said:
Can an observer tell if an archer has readied an action to "shoot the caster if he tries to cast a spell"?
As a rule of thumb, let the observer know which is the action readied (an attack with the bow, the casting of a spell, the drawing of a weapon...) but not which is the trigger.
You may vary this information depending on how you think the readying character would look like: an archer could be aiming and keeping the bow stretched (or however it says...), a caster concentrating and waving his hand in the air, a cleric touching his symbol. I think you can, depending on the circumstances, give more or less clue about the readied action, to have some variability.
A spell such as Detect Thoughts may tell you also the trigger that the readying character is having in mind.
Barcode said:
How about if a bard readies a silence spell for the same condition? Can one tell if he is readying a spell? Can one spellcraft it to try to determine what is being readied?
I would not allow Spellcraft in any case until the character has effectively started casting. Once he lets the spell go, you can identify it for counter or dispel. Incidentally, if he is readying to counterspell, not even HE knows which spell he is going to cast yet.
Barcode said:
How about counterspelling? Is it obvious that a mage is prepared to counterspell you? If not, is there at least a Spot/Sense Motive check allowed?
More complicated. I would say that you'll know that the caster is readying a "cast a spell" action, but you may not know if he is readying a counterspell or a normal spell. This may be my personal interpretation, but I don't think "counterspell" is a different action.
Barcode said:
Can the charger tell if a spear has been readied against a charge?
I really don't know how a spear should be positioned against a charge, if it would look different than for example readying to strike as soon as the opponent comes within reach. Probably a rules lawyer would say that if you want to get the double damage you must specifically ready it "against a charge" in advance.