Sense Motive (Wis)

A successful check lets you avoid being bluffed (see the Bluff skill). You can also use this skill to determine when 'something is up' (that is, something odd is going on) or to assess someone's trustworthiness.

Sense Motive DC
Sense enchantment
25 or 15
Discern secret message

Hunch: This use of the skill involves making a gut assessment of the social situation. You can get the feeling from another's behavior that something is wrong, such as when you're talking to an impostor. Alternatively, you can get the feeling that someone is trustworthy.

Sense Enchantment: You can tell that someone's behavior is being influenced by an enchantment effect (by definition, a mind-affecting effect), even if that person isn't aware of it. The usual DC is 25, but if the target is dominated (see dominate person), the DC is only 15 because of the limited range of the target's activities.

Discern Secret Message: You may use Sense Motive to detect that a hidden message is being transmitted via the Bluff skill. In this case, your Sense Motive check is opposed by the Bluff check of the character transmitting the message. For each piece of information relating to the message that you are missing, you take a -2 penalty on your Sense Motive check. If you succeed by 4 or less, you know that something hidden is being communicated, but you can't learn anything specific about its content. If you beat the DC by 5 or more, you intercept and understand the message. If you fail by 4 or less, you don't detect any hidden communication. If you fail by 5 or more, you infer some false information.

Action: Trying to gain information with Sense Motive generally takes at least 1 minute, and you could spend a whole evening trying to get a sense of the people around you.

Try Again: No, though you may make a Sense Motive check for each Bluff check made against you.

Special: A ranger gains a bonus on Sense Motive checks when using this skill against a favored enemy.

If you have the Negotiator feat, you get a +2 bonus on Sense Motive checks.

Synergy: If you have 5 or more ranks in Sense Motive, you get a +2 bonus on Diplomacy checks.

Epic Use

The character can discern a target's alignment and even detect surface thoughts.

Discern partial alignment
Discern full alignment
Detect surface thoughts

Discern Partial Alignment: This use of the skill lets a character discern one component of a target's alignment. When making the check, the character states whether he or she is trying to discern the law-chaos or good-evil component. A character can't retry the check, and he or she can't use this to discern more than one component of the alignment (but see below). The target must be visible and within 30 feet of the character.

Discern Full Alignment: This use of the skill lets a character determine both components of a target's alignment. The character can't retry the check. The target must be visible and within 30 feet of the character.

Detect Surface Thoughts: This lets a character read the surface thoughts of a single target (as the 3rd-round effect of the detect thoughts spell). There is no saving throw to resist this effect, though the target can use Bluff to disguise his or her surface thoughts (see the Bluff skill description), in which case this becomes an opposed check (any result lower than 100 automatically fails). The target must be visible and within 30 feet of the character.

Source: SRD 3.5 Combined Skills