I tackled this seemingly insurmountable task just over a year ago. I was scared to death that I would make the wrong choice, inflicting a negative experience on my child, harming him for life. I started off with one basic premise - there are plenty of good preschools. Finding the right preschool usually means throwing away preconceptions and preformed opinions, and really studying your own child. You have to, quite literally, step back, and analyze your child as an outsider would do.
Some kids have a tap of inexhaustible energy, constantly flitting from one activity to another.
If you walk into a preschool and observe that the facility doesn't give him an opportunity to run around, it's best to move on. Some kids can jump into a new school and hit the ground with both feet running - transition is not an issue. Others hold back and size up a situation before plunging ahead. In the latter case, it's critical to find a program where the teachers make an effort to involve them, while not pushing them intensely to adjust on the first day. Some kids simply take longer to make the transition.