You have a lot of experience in ferreting out the truth.
Default Attribute
Unskilled Use
You can search for clues, evidence and hidden items, but might not understand what you find.
Physical Ability
You can rank ideas in order of importance and condense them into short and pithy reports. Your prose may not win you any literary prizes, but you know how to get a main idea across to even the thickest reader. You are good at remembering things people tell you, and at taking notes. You are good at research and legwork. You are good at following mundane details such as financial and travel records and using them to reconstruct the activities of your subject. You can search an area, and be fairly certain that no hidden clues or features remain there.
You can make shrewd deductions based on physical evidence and your familiarity with human nature. You can assemble disparate clues into a mental recreation of a crime or other past event; in game terms, the GM assigns a Difficulty to the situation based on its obscurity. If you make your Investigation check, you are given a hint, or outright told what the clues mean, depending on the GM's needs in moving the story along.
You are a good judge of character and can sense when people are lying. You are a good interrogator, using psychology and the inconsistencies in what people say to pin down the truth.
Sometimes, you can't satisfy yourself with examining traces; you have to do a stakeout or shadow an actual quarry. You can Sneak using your Investigation.
Knowledge
You are familiar with police procedure and the law. You are familiar with many case histories of famous crimes. You know by reputation the famed celebrities, cops, detectives, and criminals of your day. You are acquainted with the latest forensic techniques of your juncture. You know a little about every topic under the sun. However, experts in any field you cover invariably hate the way you oversimplify complex issues and gloss over details.
Contacts
You know cops, superiors in the police hierarchy, prosecutors, civilian staffers, and (depending on your era) journalists. You know many petty criminals, including stool pigeons, as well as ordinary citizens just trying to get along. You also know reporters as well as editors, and photographers. You have contacts in every walk of life. You've met everyone from heads of state to street corner scumbags. You have a range of low-level underground contacts from whom you can get useful tips, either through bribery or intimidation. You likely have contacts among well-respected citizens and business people for whom you've done work in the past.
Unfortunately, although this skill provides a wider range of possible contacts than most any other, the people whose stories you've investigated are highly likely to despise your intestines. Note that contacts who'd like to see you squashed like a bug may, if they have some way to cause you grief, may pretend to still like you.
Skill Schticks
These are the Pulp Schticks related to this skill. Check with your GM which of them are applicable for this game.