Green’s Dictionary of Slang

(US drugs) the drawing of blood back into the syringe to make sure that one has hit a vein.

1936		D. Maurer ‘Argot of the Und.Narcotic Addict’ Pt 1 in AS XI:2 127/1: verification shot. A type of vein shot in which the blood is drawn up in the needle where the addict can watch it mix with the dope. The most rapid and satisfying type of shot for the experienced addict.
1959		J.E. Schmidt Narcotics Lingo and Lore.