Locating Creatures with Concealment
When a creature has Superior Concealment, it makes it more difficult to locate that creature with your primary senses (typically vision for humanoid creatures). When trying to locate a creature who has Concealment from you, you can attempt the following methods.
Using Spot
If a creature has visual concealment, it is impossible to detect them directly with a Spot check. However, at the GM's discretion, it may be possible to determine the creature's location due to disturbances in the environment. This task is very difficult and requires a Spot check of Difficulty 6 [DC 30] (or if the creature is using Stealth, it may use its check if higher). This Spot Check is modified by distance as normal (i.e. -1 per 10ft), and creatures of large size or larger reduce the difficult by 1 per size category (i.e. DC -5 per size). Additionally, environmental conditions may make this check easier (such as rain, standing in mud, or moving through an area filled with debris). A successful check allows you to target the Concealed creature, though you suffer the penalties to your attack roll from Concealment as normal.
Using Listen
If a creature has visual concealment, it is possible use a Listen check to determine the direction or location of a concealed creature. The DC of the Listen check is equal to the Stealth Bonus of the Concealed creature, modified by distance as normal (i.e. -1 per 10ft). If you exceed the DC, you can determine the direction in which the Concealed creature is located. If you exceed the DC by 10, you can determine the creature's precise location, allowing you to target the concealed creature, though you suffer the penalties to your attack roll from Concealment as normal.
If the concealed target is actively hiding from you (i.e. using their Stealth skill to hide), the DC is based on their Stealth skill check instead of just the Stealth Skill Bonus (i.e. they increase the DC of the check by their d20 roll).
Other Senses
It may be possible to locate a Concealed target using other senses. For example, if you can Touch the concealed creature, you automatically know the creature's location. Other senses, particularly superior senses, can be used similarly to the Listen skill.