Green’s Dictionary of Slang

(orig. US) the assessment of a place or person to calculate how vulnerable it is or they are to robbery.

1926		D. Hammett ‘Assistant Murderer’ in Nightmare Town (2001) 142: I pocket my two-fifty advance and come up here to give the ground a good casing.
1942		R. Mulvey ‘Pitchman’s Cant’ in AS XVII:1 Pt 2 Apr. 91/1: casing. Looking over the prospective customers, so that they will be able to judge to some extent the amount of business they will be able to do. ‘We put a man in the factory on the payday and while he was casing the workers we set up the flash.’.