If a spellcaster casts an illusion spell that is Duration: Concentration, and then another character comes in, can they tell that the spellcaster is concentrating on something? If so, would they be able to tell what the spell is, just as when they cast it?
There is no standard way to detect spells being maintained.
- Spellcraft only detects spells as they are being cast, not maintained.
- Knowledge (arcana) allows detecting spell effects, not the caster.
- Detect Magic is less clear, because it doesn't say if a spell aura is located on its caster or only on its effect. The Greater version does identify the caster.
However, one way to identify that someone is concentrating on a spell is using Sense Motive (hunch, DC 20). This is supported by the description of Seek Thoughts which has an additional clause raising the DC to 25.
Concentration is not obvious unless your GM says it is.
The rules neither explicitly say it is nor isn't, so GM's have leeway to decide for their campaign.
From Magic, Duration:
The spell lasts as long as you concentrate on it. Concentrating to maintain a spell is a standard action that does not provoke attacks of opportunity. Anything that could break your concentration when casting a spell can also break your concentration while you’re maintaining one, causing the spell to end. See concentration.
You can’t cast a spell while concentrating on another one. Some spells last for a short time after you cease concentrating.
No mention is made of the appearance, but we know that Concentration is not a spell's Verbal or Somatic components. No chanting or hand-waving is necessary. Some casters may be more obvious about it at a GM's discretion, such as them needing to focus on the target to the exception of their environment.
Need more proof? The rules support that a creature isn't necessarily hampered while maintaining Concentration. The list of things that can break Concentration are concise:
- Being affected by another's Spell
- Being Grappled or Pinned
- Vigorous Motion (ie riding a mount, inside a storm-tossed ship)
- Violent Motion (ie riding a galloping horse, the deck of a storm-tossed ship)
- Being Entangled
- Casting another spell*
As you can see, walking around and even talking regularly are not on this list.
*Technically, casting another spell does not break Concentration, but you must choose to break Concentration before you can cast another spell.
It is noticeable that they have a spell going.
From the Spell Manifestation FAQ:
What exactly do I identify when I’m using Spellcraft to identify a spell? Is it the components, since spell-like abilities, for instance, don’t have any? If I can only identify components, would that mean that I can’t take an attack of opportunity against someone using a spell-like ability (or spell with no verbal, somatic, or material components) or ready an action to shoot an arrow to disrupt a spell-like ability? If there’s something else, how do I know what it is?
Although this isn’t directly stated in the Core Rulebook, many elements of the game system work assuming that all spells have their own manifestations, regardless of whether or not they also produce an obvious visual effect, like fireball. You can see some examples to give you ideas of how to describe a spell’s manifestation in various pieces of art from Pathfinder products, but ultimately, the choice is up to your group, or perhaps even to the aesthetics of an individual spellcaster, to decide the exact details. Whatever the case, these manifestations are obviously magic of some kind, even to the uninitiated; this prevents spellcasters that use spell-like abilities, psychic magic, and the like from running completely amok against non-spellcasters in a non-combat situation. Special abilities exist (and more are likely to appear in Ultimate Intrigue) that specifically facilitate a spellcaster using chicanery to misdirect people from those manifestations and allow them to go unnoticed, but they will always provide an onlooker some sort of chance to detect the ruse.
This means that a casters maintaining a spell still leaves obvious traces of the spell, so a person could tell that the caster is concentrating on a spell if they can see it and the caster. They still have to make a will save in order to disbelieve it though, and if they can identify it, you may want ot give them a bonus to their will save accordingly.