I think it says it all in the subject line but the way the Core Rulebook defines concealment makes it sound like its up to the GM to decide if something gives concealment. Can someone help me on this?
Some times, it is up to the DM to just make a call. Some things explicitly provide concealment. Dim light, for instance, always provides concealment (except, of course, against things with Darkvision or other similar powers).
"Undergrowth: Vines, roots, and short bushes cover much of the ground in a forest. A space covered with light undergrowth costs 2 squares of movement to move into, and provides concealment."
"Smoke Inhalation: [...] Smoke also provides concealment to characters within it."
and a weird one...
"Invisibility: An invisible creature displaces water and leaves a visible, body-shaped “bubble” where the water was displaced. The creature still has concealment (20% miss chance), but not total concealment (50% miss chance)."
Aside from spells, It's usually easy to find out if someone has concealment against you. As a rule of thumb, if any condition causes you to have limited line of sight against someone(heavy fog, a thick bush, moving in dim light or darkness)then they would have concealment against you, or even total concealment if you have no line of sight. Some spells that grant concealment are blur, obscuring mist, fog cloud, displacement etc...
Cover represents a physical barrier, such as a wall. Concealment represents when the target cannot be seen with full clarity, such as in a severe storm.
It should be noted that you get total concealment much more often then some might think. In the case of a severe storm ("Downpour"), creatures adjacent to you have concealment while anybody further than that has total concealment.
So does that mean that concealment grants the 20% miss chance and total concealment is 50%, same as cover then?
Cover and Total Cover don't provide any miss chance, only concealment does. Cover DOES provide a +4 bonus to AC against opponents you have cover from, and a +2 bonus to reflex saves from burst effects. total cover means the person with whom you have total cover against can't attack you at all.
In the military, we learned that the difference between "cover" and "concealment" is actually pretty simple...both block vision, but "cover" will also stop a bullet.
Unfortunately it doesn't exactly work this way in game. Great to think of it that way and it makes it easier to visualize, but you will never accidentally shoot an arrow into a creature standing between you and your target.