He did trained with Bruce Lee, but Steve McQueen was a very tough street smart guy long before he and Bruce Lee met. McQueen because of a really bad home life grew up on streets running with gangs until one day he got in such trouble after many run in with the law that the judge at his hearing gave him a choice - prison or the US Marine Corp. He chose the USMC. Back then, it was not unusual for courts to do this.
I don't think he knew a martial art in the sense that he studied it seriously with the intent to master it. But he did know how to fight.
No he didn't know martial arts , boxed as a kid but never learned the arts .
Yes he boxed when he was younger and he also learned Tang Soo do
He did have a friendship with Bruce Lee if not trained by Bruce Lee directly,he may have absorbed of his knowledge indirectly
I know he knew Bruce Lee
