Probably going to the Breed Associations websites would be your best starting place. From there you can find nearby breeders whom you can visit, talk to.
Not sure exactly what you want to know. The Association is big on genetic testing for proof of sire, carriers of color and short legged genes. This is all visible on the registration paperwork. Bulls are blood typed and advertised so you know what they carry to breed to your female cattle. There are some lines coming in polled, a good trait to breed in.
If you are wanting just small cattle, the Dexters are an old breed, set standards of expectations. In this, they have a bit of advantage over other crossbreds that are appearing on the market. Few surprises because of set genetics.
I had a Dexter heifer, she was easy on the land, trainable, very easy keeper gaining on pasture only. Seller told me her family was all good mothers, never had any problems taking care of their calves. They liked the beef of their Dexter steers. Of course any cattle handle better if you start doing it very young. Mine sure loved her daily routine, but would accept change without too much fuss. She was kept with our 4-H calf. Dexters are "real" cattle in thinking, reactions, and long-legged ones not tiny at 700-800 lbs., though much smaller than traditional mature cattle.
The ADCA is a good place to start. There have also been a few magazine articles written that you can probably google and find. The most recent was Hobby Farm, and I seem to recall Mother Earth News did one a few years ago.
We've been breeding for a few years now, and are up to nearly 30 head. We have a polled bull, but do appreciate the beauty of the horns on the cows. We're breeding now for polled stock, and short-legs...we'll keep most of our original cows simply because we're fond of them....there are pros and cons to horns/polled, long-legs vs short legs. We chose Dexters for all the reasons you hear about - easy to handle, smaller size (we're just 5'4", and nearly 60, this was important to us), great mothers with little difficulty calving, do well on pasture, excellent beef. We've found all that to be true, really. I would suggest visiting a few breeders if you haven't already. Hope this helps!!