Stan:

He gets like really reddish green, and like he can't look at you or he'll vomit (inna good way :)

Kyle:

He get's like all red and flustered, and he kind of stutters

Eric:

he's like actually insults you....

Kenny:

He like plays with his fingers and he like laughs a lot

Craig:

He tries to hide that he's blushing but fails and he tells you to shut up a lot

Tweek:

...he drops things...when he's not holding anything...his hands are SHAKING

Token:

He like rubs the back of his neck alot

Clyde:

He like is brutally honest, like "oh my god your ass is SOOO tasty!" honest

Jimmy:

He actually gets like super shy and hes a blushing mess

Butters:

Butters? more like Stutters! {i laughed at my joke you should too}