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.