We don't know, but a lot of readers have theorized she put it together based on her basic Tower knowledge about the Forsaken (Sammael has a scar, he's blond and short - and as we know he didn't use Mask of Mirrors, so it's how he looked as Lord Brend) and what she gathered from Loial. It sounds extremely likely she managed to get a look a Sammael, or to get a good physical description from someone who saw him. She was already almost convinced a Forsaken was in Illian, and could inquire about dreams, or listen to stories about those etc.