So from the fluff that I've read, a run of the mill Farseer will find big event that effect their own craftworld, such as it being attacked and then try to work out what events are the trigger points for that event happening. It is not fool proof, such as the events that happen in the last "Path of" book. Once they find the trigger points, they play the what if game and try to find the best way to stop it.
Significant Farseers like Eldrad go further ahead in time, but they are still looking for big events.
There is a quirk in craftworld Eldar, while there is the large survival of the Eldar race thing happening, they will put their own Craftworld first.
For example, Ulthewe may out right ignore a Nid/Orc infestation if it was no direct threat to them, even if it might end up trashing Alaitoc. They might put in a call to Alaitoc, and say hey guys did you notice this.. you guys might want to take care of them. It is also possible that Alaitoc's actions might redirect the threat to another craftworld, not so much out of Malice, but it would mean less Alaitoc lives lost.
So yeah Eldar are not really like Minority report, trying to fight crime at a low level, there just isn't enough of them for that sort of thing. If a cult would eventually become a problem for a craftworld, then it would see if it could be redirected to another race to minimize the loss of Eldar lives, if no other option, then they would take them out directly. It's all about survival for CWE.