Determining a creature's HD

[Yogibear41]

Is there a way in the rules to determine a creature's HD? For example say I have an ability and I need a creature of 15 or more HD for it to work, other than opening a MM and finding a creature with 15 HD is there some way I can determine if a particular creature has X amount of HD?

[Thurbane]

Make the relevant Knowledge check and reverse engineer the result? Base DC is 10 + HD to get a piece of information, plus an extra piece for every 5 you exceed the check by.

I also think there's a feat or skill use somewhere (maybe using Sense Motive) that allows you to judge the BAB of an opponent; maybe in Complete Warrior?

[eggynack]

The first level druid spell power sight from MotW gives you the target's HD count.

[BowStreetRunner]

Are you looking for something in-game, or out-of-game? Because my initial reaction was to assume you are looking for a table of monsters by HD.

[PunBlake]

I also think there's a feat or skill use somewhere (maybe using Sense Motive) that allows you to judge the BAB of an opponent; maybe in Complete Warrior?
I also remembered something like BAB and CWar, but I didn't find it. Instead, I found a use of Sense Motive called Assess Opponent (CAdv). It's vs Bluff, within 30', and (unfortunately) compares you to your opponent's CR instead of HD. The feat Combat Intuition improves this.
Knowledge checks are more helpful for this specific question, but combining the two could maybe improve accuracy.
I was thinking the Know Opponent spell (SpC) could give you HD, but no. The spell Know Greatest Enemy (SpC) gives a 60' cone of, "What's the highest CR thing here?" which could be of situational help.
You could probably use various Divination spells for yes/no questions and ask if the thing you're looking at is appropriate for your ability.

[flappeercraft]

The Detect Alignment spells can tell you it's within a range although not exactly.

Also Trap the soul has this line.

"Material Component
Before the actual casting of trap the soul, you must procure a gem of at least 1,000 gp value for every Hit Die possessed by the creature to be trapped. If the gem is not valuable enough, it shatters when the entrapment is attempted. (While creatures have no concept of level or Hit Dice as such, the value of the gem needed to trap an individual can be researched. Remember that this value can change over time as creatures gain more Hit Dice.)"

[Psyren]

I also think there's a feat or skill use somewhere (maybe using Sense Motive) that allows you to judge the BAB of an opponent; maybe in Complete Warrior?

Rules Compendium has "Assess Opponent" as a use of Sense Motive (RC 68). It helps you gauge CR though rather than HD.

[DeTess]

As a DM, I'd just give people the information if they need it for a feature or spell. So I'd tell them the simulacrum they want to build of creature X will cost them 2000 GP, or that the creature is strong enough or weak enough for a particular feature to work on. Since the class feature calls these game-terms out, it'd be odd if the one using the feature wouldn't be able to tell a valid target from a non-valid one.

[Kelb_Panthera]

OA has rules for using sense motive in an iaijutsu duel that give you the opponent's level (pretty close to HD in most cases), ranks in iaijutsu focus, and total attack and damage bonuses with their primary weapon.