I don't know if you can tell if a mare has had a foal just by looking at her, but my mare was a broodmare before I got her and you can definitely tell with her. She has the saggy belly that never totally tucks up nicely and she has saggy boobs. They are huge.
So I DO think if they've had several foals you can probably tell.
Does your girl have an prominent udder?
Here is mine. She's 18 yrs old and has had 10 foals (not my doing).