Green’s Dictionary of Slang

an inspection of a client’s penis made by a prostitute before intercourse.

1972		B. Rodgers Queens’ Vernacular 208: washing a man’s penis before coitus affording detection of any veneral dripping [...] precheck.
1986–7		Maledicta IX 150: The original argot of prostitution includes some words and phrases which have gained wider currency and some which have not […] precheck (short-arm inspection before intercourse).