I am very new to breeding minis, but here is my experience. My regular vet does not have experience ultrasounding minis, so I have used another local vet who does. She was able to tell me in a few seconds that "there is your baby" and even gave me photos. I suggest looking for a vet that has the equipment and expertise with minis. I have also used the Pregnamare blood test kits which can be used after about 40 days. Someone on the forum sells them (about $40 each - much cheaper than a vet call!) or try Google. We have one mare that was recently blood tested pregnant and will be ultrasounded in a couple of weeks so I can compare results.
BTW, we had a false pregnancy this year, so we plan to repeat the ultrasounds later to make sure the mares are still pregnant.