With my experience, there really no tell tell signs that an anemone is about to split except on the day its going to do it.
Mine has spit 3 times in 5 years ( around this time of the year ) and each time during the day it will move back behind its rock and during that night it splits.
Next day or so they move back out in front for the light and takes a week or two to heal up nice.
Maybe others act different. IDK