Back when I got Frosty, she been with a Stallion and she was so fat that I thought she was pregnant. Not wanting to take any chances, I got the vet out and he draw blood and had it tested. Turned out that she wasn't pregnant.

As far as I know there are three ways to determine if a mare is pregnant.
- The most commonly used method is to do a blood test.
- The more reliable method is to do an ultrasound
- Then it is also possible to stick your hand into the womb and feel for a foetus. Now this might sound like something horse owners can do themselves, DON'T... There is a high degree of risk involved. From infection to injuring the foetus. I believe even a vet will try to avoid this method as far as possible.
So to answer your question, I don't think so, unless the pregnancy is well on its way and the mare's udder start swelling and milk is produced. On the other hand, the owner who know his/her horse really well, might be able to pick it up from the horses behaviour that something is up.
As for riding a pregnant mare. If she is
fit (emphasis on fit) and healthy one could ride her
gently up to 8 or 9 months into the pregnancy and the exercise will probably do her good. However if there is even the slightest doubt in your mind, then rather don't ride her and let her rest.