This is unaddressed by the rules. In fact, the Pathfinder Spellcraft skill description of the skill use Identify a spell as its being cast is as spare as the D&D 3.5e SRD's Spellcraft skill description of the identical task. Further, while how to employ the skill Spellcraft to identify a spell in the latter game is given a more detailed treatment in Tome and Blood (July 2001) by Monster Manual (2000) author Skip Williams, even this expanded treatment of this Spellcraft skill use still fails to explain the depth of the creature's understanding upon successfully identifying a spell as it's cast. (Much in the same way the Pathfinder FAQ does in this entry.)
Thus it's up to the GM how much information a caster reveals about a spell as the spell's being cast.
In this GM's campaigns, only the spell is identified, not the caster's choices. To get more information about the spell requires making a Knowledge (arcana) skill check to Identify a spell effect that is in place after the spell's cast. In play, this means I usually just let the player read the spell's description when his PC successfully identifies a spell, but successfully identifying the spell doesn't give the PC more information than what the PC could determine from witnessing the spell's subsequent effect.
For example, a PC that successfully identifies as it's cast a teleport spell doesn't learn of the caster's intended or actual destination, but a PC that identifies as it's cast a fireball spell that afterward detonates in his party's midst can likely infer that fireball spell's picked point of origin.
Similarly, for example, a PC that makes a Spellcraft skill check to identify as it's cast the spell resist energy (DC 17) knows that the troll shaman can typically pick from acid, cold, electricity, fire, or sonic, but the PC must subsequently make a Knowledge (arcana) skill check to Identify a spell effect in place (DC 22) to know that the spell granted the troll fire resistance. (Alternatively, the PC may guess at the resist energy spell's effect after his burning hands spell had a reduced effect on the troll shaman.)
In the specific case of the spell plane shift, the spell's focus component—"a forked metal rod attuned to the plane of travel" (discussed at length in answers to this question and this question)—is a clear indicator of at least part of the spell's effect. That is, the focus is a clear indicator of the spell's effect if such forks are easily recognized as different from one other! If they are, a GM may allow an observer to determine the plane to which the caster's shifting either from the initial Spellcraft skill check used to identify the spell as it's being cast or with an additional Spellcraft skill check with an arbitrary DC. However, a GM may instead rule it impossible to identify a plane by fork alone, saying, for example, that all such forked rods appear identical or that casters of plane shift deliberately conceal their planar destinations by grabbing all their rods from their spell component pouches and mentally designating only one! Either way, even observers that identify broadly a plane shift spell's destination (e.g. Mechanus or the Nine Hells of Baator) should remain ignorant of the caster's precise intended planar destination (e.g. the Fortress of Disciplined Enlightenment or God Street, respectively).