To have essential and current information; understand what is important: You look like a smart lad who knows the score(1940s+)
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with know the score
know the score
Also,know what's what. Understand what is happening; be familiar with the real story or the full situation. For example, It will take the new legislators some time to know the score, or When it comes to teaching youngsters to read, Nell knows what's what. The first expression, dating from about 1930, alludes to score as a tally of points in a game. The variant dates from about 1400.