Green’s Dictionary of Slang

to see, to look at.

1856		C.W. ‘Tiger-Slayers’ in Sporting Rev. Mar. 207: The mahout pointed towards the dark copsewood, and cried out, ‘Dekh, sahib! dekh!’ (‘Look, sir! look!’).
1933		M. Marshall Tramp-Royal on the Toby 95: I dekk the long row of hotels and boarding houses on the promenade.
1995		J. Cameron Vinnie Got Blown Away 98: He put a mitt on her shoulder let the grizzlies deck she was his.
1999		Indep. Rev. 18 Dec. 10: So me and Ernie decked this tosser in a Millwall scarf and jumped on his head.
1999		G. Seal Lingo 119: More terms concerned with the pugilistic include [...] to be decked, as in i should’ve decked the bastard, deck being another Americanism that we have made our own. (Though deck or decko meaning to look does seem to be one of ours.).