Fo sho. Lots of landscapers use the 3', 4', 5' shortcut to layout square corners where they're needed, like on hardscapes or beds. Measure 3' one direction, 4' in a right angle, and confirm the angle with a 5' hypotenuse between the ends of the two.
Piece of pie.
I saw a guy do it putting up a pole barn one time also.