Moose, McEnroe was partially correct. Crossing the net to look at a ball mark is a code violation, but not a default.
As to those arguing wit moose about the old time umpires checking ball mars, moose is correct.
Back then, if the player wanted a mark checked, he had to ask the line umpire. A lot of the old school linesmen didn't want to be shown as wrong, so they would only look at a mark if they knew they would be right.
If the player argued with the linesman, and the linesman was unsure, the only way a chair umpire could go look is if the linesman yielded to the chair.
Then it evolved to where the chair could go after the linesman if they were fairly certain the linesman went to the wrong mark.
Then thankfully it evolved to the way it is today.