What is the term for someone who provides robbers with the information on the victim?
Solution 1:
'Inside man' is the expression with which I am familiar in this context.
The Ngram indicates that this fairly well known expression 'inside man' is used at least half as much as the expression 'inside job'
Inside job - a crime, especially stealing, committed by someone in the place where they work
Cambridge
Strangely, I could not find 'inside man' in any dictionary that I checked, even the Urban Dictionary which has the words but with different meanings.
But there is a well known film dealing with the meaning called 'Inside Man' see Wikipedia.
The term 'insider' has a large spectrum of meaning - see Wikipedia - including whistle-blowing. 'Insider' is only used in one criminal context that I can find (see the Wikipedia article again) and that is 'insider trading' on the Stock Market.
Solution 2:
As in:
The informer provided the robbers with information on the victim?
informer
- a person who informs against someone, esp a criminal
- a person who provides information: "He was the President's financial informer".
Solution 3:
Good question! In the US the term probably originated in the early 20th c along with words like grifter, confidence man, hustler, and so on. Related usages include short con and long con. There are more than a few films which describe the roles played in each type of scam including Robert Bresson's brilliant Pickpocket, Mamet's House of Games, Frears' The Grifters, etc., but Bresson's film is the most detailed and specific.
I wasn't able to identify a single word for this role in a crime but the Urban Dictionary had suggestions including lookout and ventana.
* Addendum *
Elizabethan English is a veritable treasure trove of idiomatic expressions from the underworld. A recent book which is worth plumbing is Arthur Kinney's 1990, Rogues, Vagabonds, and Sturdy Beggars: A New Gallery of Tudor and Early Stuart Rogue Literature Exposing the Lives, Times, and Cozening Tricks of the Elizabethan Underworld, e.g.,
The vitality and exuberance of Elizabethan life is nowhere better seen and enjoyed than in the rogue books and cony-catching pamphlets of the sixteenth and early seventeenth centuries. What a colorful and witty crowd of ragamuffins and renegades these Elizabethan rogues and vagabonds are: cutpurses, hookers, palliards, clapperdudgeons, dummerers, jarkmen, doxies, and crosbiters; counterfeit cranks, abraham men, bawdy-baskets, walking morts, kinchin cos, and priggers of prancers; minstrels, gypsies, peddlers, tinkers, jugglers, and bearwards....following Barnard's Law or Vincent's Law, they created their own guilds with their confederates. Little wonder, then, at Falstaff's easy familiarity with the life of the petty criminal, cozeners and cheats. p. 14, to understand the growth of Tudor vagands, we must sort out both the historic causes -- the enclosure movement, the dissolution of monasteries -- and the more immediate sources, such as the disbanding of professional soldiers and the arrival, from western Europe, of gypsies or "Moon-men", the introduction of piece-work production, a sharp increase in population, inflation, and debased money...each leading to unemployment which became chronic early in the sixteenth century, spreading throughout the Commonwealth.
Among the literature Greene discusses is Robert Greene's Blacke Bookes Messenger, 1592, which denounces 'conny-catchers' (card sharps), cutpurses, nips, foysts, light-fingered gentry, dishonest colliers, horse thieves, pickers of locks and other rogues, villaines and coosenages.
I'm sure that these books would have reference to the specific word you seek. From there it shouldn't be too difficult to find their modern analogues in the OED.
* Another Addendum *
This book The Big Con: The Story of the Confidence Man by David Maurer, 1940, surely contains the answer to the OPs query. The focus of the book is linguistic. Just so terms like roper and steerer are consonant with the OPs query. Here's the classic structure to most if not all cons as described by Maurer:
All confidence games, big and little, have certain similar underlying principles; all of them progress through certain fundamental stages to an inevitable conclusion; while these stages or steps may vary widely in detail from type to type of game, the principles upon which they are based remain the same and are immediately recognizable. In the big-con games the steps are these: 1. Locating and investigating a well-to-do victim. (Putting the mark up.) 2. Gaining the victim’s confidence. confidence. (Playing the con for him.) 3. Steering him to meet the insideman. (Roping the mark.) 4. Permitting the insideman to show him how he can make a large amount of money dishonestly. (Telling him the tale.) 5. Allowing the victim to make a substantial profit. (Giving him the convincer.) 6. Determining exactly how much he will invest. (Giving him the breakdown.) 7. Sending him home for this amount of money. (Putting him on the send.) 8. Playing him against a big store and fleecing him. (Taking off the touch.) 9. Getting him out of the way as quietly as possible. (Blowing him off.) 10. Forestalling action by the law. (Putting in the fix.)
Solution 4:
Although it's obviously not a single word and it doesn’t always mean someone whose sole involvement is providing information, I would use:
“the person [guy/accomplice/accessory] who cased the bank [joint/job].”
(from Glannon Guide to Criminal Law: Learning Criminal Law Through Multiple-Choice Questions and Analysis, by Laurie L. Levenson, via GoogleBooks)
For a slightly shorter, yet still multi-word possibility, the notion of "the/a thief's/gang's/mob's advance man" could be considered.
(from American Poultry Journal, also via GoogleBooks)
For a single word, from the admittedly questionable world of crossword puzzles, "caser" has occasionally been offered as “maybe” the single-word “answer” to clues such as “Burglar's/Bank robber’s advance man,” which actually gave rise to this question & answer on Stackexchange.com’s ELL site.