Thanks for the question and thanks for using JustAnswer.
This question doesn't really have a good black and white answer. Most of the time the answer is never. Most mare owners never really feel the foal moving in the womb. That's not to say it isn't possible, just not usually noticeable except to the very astute. But at the same time I have seen mares carrying very active fetal foals who kick and squirm all the time and the movements are very noticeable and visible.
I think in some mares and some owners you may see a foot or a kick on her side around half way through gestation. Since a horse is pregnant for around 11 months that would be around 6-7 months.
This is sure not a good way to diagnose pregnancy or tell how far along a horse it - just something that is very exciting when you do get to feel or see the baby moving.
I hope this kind of vague answer answers your question.