(idiomatic, US) to vet or evaluate; to criticize or correct, often by hindsight, by presuming to have a better idea, method, etc.
Example: 1946, someone, somewhere:
I suppose anybody who keeps a diary and subsequently goes over it for publication has a tremendous temptation to second-guess and make himself look like an oracle.
1957, United States Senate proceedings:
As a practical matter, a fertilizer company could not afford to second-guess the Federal Trade Commission or a jury in a triple damage case on so obscure a point.
1959, U.S. Court of Appeals proceedings:
Public administration would be hamstrung if courts were free to second-guess reasonable administrative decisions.
1995, someone, somewhere:
MacGregor avoided this trap by refusing to give managers reporting to him the opportunity to second-guess the solution he would be most likely to choose.
2003, Editing Today:
If you suspect you've stepped over the line, ask a few other copy editors to second-guess your headline.
Please don't try to second-guess the procedure that we have already refined and adopted.
Once she began listening to her instincts and didn't second-guess herself the entire time, her artwork improved noticeably.