I also say multiple options.. altho a bit different that Liquid Sabre. TO me it more depends on what part the NPC has to play in the scene.
If the NPC is acting as the point of contact..'face man'.. for the underworld, use Gather Information. Rather than seeking out the information yourself, you are relying on the network already built. In this case he is already an indentified freindly, but the right questions have to be asked as he won't just blurt out all the info he has acquired.
If the NPC is a character that belongs to the underworld, but not a specified 'face man' for the network, then its the Intimidate and/or Diplomancy {with possible Bluff's thrown in based on what is said} . In this case he is potentially unfreindly and needs to be talked into providing the information.
D20 RAW does not really get very detailed with social 'combat' like this, so its really hard to do this without adding mechanics to the rules. Unless you want a single die roll to cover what could be a gret roleplaying opportunity.