In Wheel of time, CoT, we see Egwene is almost continuously plagued by headaches. From the text we can safely assume that the headaches were caused by
Halima, who was in fact Aran'gar.
Since the headaches were caused by Saidin, they were undetectable but their timing can easily help conclude the culprit.
However, what was the reason for causing the headaches? The only effects I can surmise are
- Egwene seems cranky and snaps at her advisors. Which keeps Salidar Aes Sedai divided
- Egwene trusts the culprit and relies on her sometimes disbelieving rumors about them
- The most important but I don't know why it is so important for Darkfriends that Egwene can't remember her dreams, except the feeling they were about something "dark".