This was actually a question asked to team Jordan, and Jordan never confirms this as the question was asked after his death. However, those of you who have read the Wheel of Time know that Jordan isn't the only source of canon: the members of his team, editors, readers, secondary author (Brandon Sanderson) are also considered canon when speaking about the books after Jordan's death. From an interview in 2013, Maria Simons (a major player on team Jordan whom Brandon Sanderson consulted throughout the last 3 books) says this,36.
TEREZ How did Moiraine identify Sammael and Be'lal? How did she know
MARIA SIMONS I'm assuming research; I don't know.
TEREZ My theory was always eavesdropping.
MARIA SIMONS Oh yeah, that's another good possibility.
TEREZ And that would be a reason why she tipped Sammael off to her
MARIA SIMONS Yeah.
TEREZ How did she rediscover balefire? Assuming research again?
MARIA SIMONS Assuming.
MARIE CURIE That question always comes up, about whether you can learn
a weave by reading about it.
MARIA SIMONS Yeah, I think it can help, and then you have to
experiment and hope for the best.
FOOTNOTE Maria answered the question about Sammael again later.
From an interview with Brandon Sanderson
LUCKERS How did Moiraine identify Sammael and Be’lal in The Dragon
BRANDON SANDERSON I have no idea.
So all we know for sure is that Moiraine knows. But remember she does snoop, and is found out and that's why they make a run for it. So I'd say between her personal research - which was massive, as she knew some of the real names for the Forsaken as well as having some physical descriptions (remember Sammael wasn't hiding under an illusion and neither was Rahvin) - and the fact that her eavesdropping ability is a well known favorite of hers, we can assume that she put clues together to figure out who Lord Brend (aka Sammael) was.
Personal hypothesis, is that she had found a physical description of Sammael, and we know he wasnt under illusion so simply asking people what Lord Brend looked like would have let her know it was Sammael, that and the fact he had Darkhounds at his call.