Obviously you have to take into consideration the body shape too - some mares never loose their stretched abdomen and this can be particularly noticeable if the mare has not been in work. Look under her tail at her external genitalia where you may see scarring from previous tears or stitching and again things might be rather shapeless from foaling. This is your conundrum sobest to get some expert 'on-site' advice from a vet who specialises in equine breeding, i.e. a stud vet if there is one near you. There is no substitute for experience. Hope this helps and please send me an email to let me know how you get on.
Feedback is always interesting and useful.