One of the players of the oneshot campaign that I'm writing chose the feat **Pierce Magical Concealment** (Complete Arcane, p.81), as a DM I'm not sure how it would work in an encounter I am planning.

The manual states about the Pierce Magical Concealment feat:

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> You ignore the miss chance provided by certain magical effects.
> 
> 
> Your fierce contempt for magic allows you to disregard the miss chance granted by spells or spell-like abilities such as *darkness*, *blur*, *invisibility*, *obscuring mist*, *ghostform* (see page 109), and spells when used to create concealment effects (such as a wizard using *permanent image* to fill a corridor with illusory fire and smoke). In addition, when facing a creature protected by *mirror image*, you can immediately pick out the real creature from its figments. Your ability to ignore the miss chance granted by magical concealment doesn't grant you any ability to ignore nonmagical concealment (so you would still have a 20% miss chance against an invisible creature hiding in fog, for example).
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> 
> 

This doesn't specify if a character with this feat can actually **see** an enemy that is, for example, under the effect of the spell **Invisibility** (Player's Handbook, p. 245).

I am planning an encounter in which the party will be invited by a wizard for dinner, an assassin under the effect of invisibility will pretend to be an **Unseen Servant** (Player's Handbook, p. 297) until the wizard gives him the signal to attack the party.

Will the character that has the **Pierce Magical Concealment** feat be able to see the assassin, or at least have any advantage in noticing he is not an Unseen Servant?
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The feat indeed doesn't specify if the character can or cannot see its target and the final decision would be up to the GM. However we still have some pieces of information to use to try to find a more satisfying answer.

If you proceed logically, the 50% miss chance when attacking an invisible target results from the Total Concealment, which implies that the character doesn't attack directly the creature, but the square it occupies :

> 
> You can’t attack an opponent that has total concealment, though you
>  can attack into a square that you think he occupies. A successful
>  attack into a square occupied by an enemy with total concealment has a
>  50% miss chance
> 
> 
> 

Therefore ignoring these 50% miss chance would imply that the character doesn't attack the square anymore but directly the creature and so has a way to distinguish it. Note that being able to distinguish the invisible creature doesn't necessarily involve that it can see it properly. For your example, the character could be able to see the assassin as a shapeless form, being enough to target it if he wants to attack him, but not enough to be able to say if it really is an Unseen servant or not (especially as the Unseen servant is described as a *shapeless force*).

By this way, you can create a clear difference between this feat and other features that are clearly designed to allow to see the invisible like the See Invisibility spell, avoiding to make such features obsolete because of this feat.
No. 

You can negate the miss chance for concealment, but that doesn't mean you can see invisibility. It is not explained how this works, so there is no textual basis for believing you could distinguish between an unseen servant and an assassin. Perhaps you can see a glowing interface layer, perhaps there is some distortion at the creature boundary, perhaps you're mildly precognitive. 

On the other hand, you probably do need opposed disguise/spot checks for the assassin to imitate an invisible stalker (likely with a very large bonus for the assassin). Were I running that game, I would be inclined to allow a generic circumstance bonus to those spot checks for the ability to Pierce Magical Concealment.