Green’s Dictionary of Slang

chalk off v.2

also chalk
[one stares at a metaphorical chalk mark]

to look at closely.

[UK]‘Ducange Anglicus’ Vulgar Tongue 7: Chalk v. To chalk him off. To observe a person attentively so as to remember him.
[US]Matsell Vocabulum 18: chalk To Spot.
[US]C.E. Mulford Bar-20 i: Johnny used to chalk off when he hugged her, usen’t yu, Johnny?