Typically registration for a regular school-year preschool (not in a daycare center) is in January if you want to start the following September in my area. Try to call and schedule visits in October and early November if you want to get a good feel for how the kids and teachers relate to each other. The kids should be adjusted to being in school and know the routine by this point, but it's before everything starts to get crazy for the holidays.
If you're planning to start your daughter in preschool when she turns three, the best time to visit is the fall before she goes because there is a high turnover in early childhood positions and you want to meet the director and teachers who will be there when your daughter goes.
Talk to the moms in your neighborhood and see which schools they like and why. You want to find one that is the best fit for your daughter, not just the one that is supposedly the best in the area. Preschool age kids still learn best through playing, so I would steer clear of any program that seems overly academic or competitive. That's too much pressure to put on a child that young.
Relax and enjoy your time with your daughter while she's still a baby. You have plenty of time to find the right preschool for her, and she will have 18 or so years to go to school.