So How Good was Marvin Hagler Exactly?

[THE THIRD MAN]

I have to be honest, I know absolutely nothing about Carlos Monzon as a fighter :/
I should make the effort!

But goin back to Hagler... When roughly did it become common knowledge (or at least when was it suspected) that he was a right handed southpaw?

I did see somewhere that Angelo Dundee asked a kid to go and get Hagler's autograph before the Leonard fight, he signed it right handed! He was a switcher, but fought predominantly as a southpaw most of the time.