Green’s Dictionary of Slang

cool v.1

[backsl.; thus cool him, look at him, ‘a phrase frequently used when one costermonger warns another of the approach of a policeman’ (Hotten, 1867); note: cite 1683 fits the sense, but may suggest a cool glance, and is unlikely to be backsl]

to look at; esp. in cool the esclop! look, the police!

[UK]London Jilt pt 1 109: Perhaps that my Face was not disagreeable to him; for he cooled me at a distance, and indeed I could not forbear looking at him .
[UK]Hotten Dict. of Modern Sl. etc.
[UK](con. 1840s–50s) H. Mayhew London Labour and London Poor I 23/2: Cool the esclop ... Look at the police.
[UK]Wild Boys of London I 150/1: ‘Cool him,’ said Ikey Bob; ‘ain’t he getting big?’.
[US]N.S. Dodge ‘Vagrants and Vagrancy’ in Appleton’s Journal (N.Y.) 6 Sept 308: Cool the esclop (look for police) is almost the only vagrant phrase with which any of the constabulary force become familiar.
[UK] ‘Thieves’ Sl.’ Gent.’s Mag. CCLXXXI Oct. 348: ‘Cool the delo gum’ would, if used by a coster to another, convey a friendly warning to keep an eye on a person who is ‘no good’ from a business point of view. Literally translated it is ‘Look at the old mug’.
[UK] press cutting in J. Ware Passing Eng. of the Victorian Era 91/1: Cool her on Sunday in a black velvet costoom, with boots, gloves, and gamp to match.
[UK]D. Powis Signs of Crime 179: Cool [...] (b) look (backslang, ‘take a cool at that’).