The only way to get a good clue of gestational age, if you don't have a breeding date, is an ultrasound. This is what I ended up doing with my mare Freyja, last year, as was recommended to me by Tealamutt, another forum member here. Freyja was pasture bred and the seller had no clue she had even been re-bred, so there was no clear idea on dates. I got her U/S to get a gestational age, because she was showing a lot of signs (relaxation of the tail head, udder development, etc) and the ultrasound showed her to be only around day 295...she just showed signs early. She actually went on to carry 360 days before foaling, even with the early signs. I'd recommend getting an U/S to get a clear gestational age of the foal, if the breeding date isn't known.