she won't be able to calmly sit in front of you, while steadily making eye contact and not fidgeting, if she's on crack or meth. if it's meth she may be itching her arms some too. either way she'd be constantly looking away and hoping to escape for more soon.
Invite them both down for dinner, insist on it. that way she'll be stuck w ya for at least 3o minutes or so to speak with. try not to give em much notice, you'll have a better chance of catching her zonked. then if you're sure she's wacky tell your bro she must go. good luck