How does Laertes already know she’s mad? She hasn’t SAID anything yet. She must have a look about her or she must be doing something that shouts “I’m mad!” I don’t think anyone would have had time to tell him before – the madness is relatively new and he’s only just arrived. Whatever she’s doing, however she’s looking, she must look obviously crazy otherwise this would be an odd assumption to make.
It’s a heavy madness, too, it would seem – one that it will take a lot of weight to even out on the scale of his inauguration/revenge.
