The usual signs, bagging up, loose back end. If you tap along the sides of their tail head and the top of her backend it will be loose almost like jello. Her vulva will become loose/relaxed as well.
Her hips will start to stick out, usually a hollow will appear in front of the hips as the foal drops in preparation for birth.
Some donks become ravanous before birthing. I have one donk who eats like crazy 3 days before foaling, everytime. Some will go off feed. Some will start hanging all by themselves if not alone already.
Some act a bit depressed, I think they are just feeling the beginning of labor when they act that way.
Little wax beads on the ends of her teats might appear.
Donkeys are pregnant for usually 1 year.
I don't know if that helped or not-but now you have more signs to look for