I don't think the timing exactly works for this, but Elayne's knock-off medallions? Since you can't channel while it's touching you, just form a line of sisters and have them put it on one by one. Mesaana's mask of mirrors will fail, and she won't be able to channel a lick to help herself.

Failing that, you could do much the same with a hefty dose of forkroot. Have sisters take it in batches, and when they keel over you can start slicing away to find the person under the mask of mirrors.

Though actually if the Amyrlin were anyone other than Egwene, an a'dam would be the way to go. You could have the sister reswear the oaths and make sure she wasn't channeling anything but the thread of spirit. And if she does try anything hinky, she's already caught.

I'm surprised there isn't some way Aes Sedai have to tell if someone is a Forsaken. They can detect shadowspawn, right? And while ordinary darkfriends might be able to pass under the radar, someone with a direct and powerful connection to the Dark One ought to be just as noticable. It should surely be possible to create a ter'angreal that can do so - if a'dam can read your mind, so can other ter'angreal.