Green’s Dictionary of Slang

1. (US Und.) in a pickpocketing gang, the member who surveys the street for approaching policemen; any form of lookout.

1882		Nat. Police Gaz. (NY) 1 Apr. 7/1: ‘You thought she was all right but I tell you what, she was a decker!’ ‘Decker?’ said the chief, looking about, "what does that mean?’.
1925		H. Leverage ‘Dict. Und.’ in Flynn’s mag. cited in Partridge DU (1949).

2. (Aus.) a glance.

1951		D. Stivens Jimmy Brockett 83: ‘Nice office you’ve got here, Jimmy,’ he says, taking a decker round the room.
1978		V.G. O’Sullivan Boy, The Bridge, The River 67: Give us a decker at yer war wound.