It depends on what you are looking for in a preschool.
If you are looking for a daycare that teaches a preschool curriculum, that's one thing. If you need daycare now, look for one now that has a preschool curriculum and she will transition into the program when the other kids she's in care with now. She will benefit from the consistency....when looking for a daycare with a preschool program, don't assume that means a center. Lots of home daycares do a preschool program that is as good or better than any center. AND if you do have your child in a good daycare, a separate preschool won't help, it may actually be a bad thing...it depends on how well the daycare does a preschool program.
If you don't need daycare, you are just looking for a jumpstart on kindergarten, preschool may or may not be the answer. All the preschools in the world don't mean a thing if you don't interact with your child, read books, talk to them about the world around them. If you put your mind and a little effort to it, you can do better on an informal basis, than any preschool can do in a formal setting. Again, consistency and a familiar setting is a good thing for children that young.
If you are looking for a preschool because you feel your child needs some other interactions besides what you give her, be it the interaction of her peers or other adults or because she seems to have moved beyond the routine parent-child interaction and you don't know what else to do with her or feel you you are out of your depth, or if she has a special need that would benefit from more professional attention, then I would definitely say look at preschools.
If she does seem to have a special need, if she seems in anyway to be developing slower than she should, talk to your doctor and get her evaluated by the health department....if she needs help, she can get help NOW and also have an automatic "in" to public preschool.
If after reading all this, you want to find a preschool program, go ahead and start looking now. Most of them won't be doing waiting lists for a 1-1/2 year old when the program doesn't start until age 3. Only daycare centers do that and if you are not in need of daycare because you work, go to school, etc., don't put your child in an institution like that. That is NOT the place for her. What you want is a partial day program, maybe 2-3 days a week. Not an all day program, with mass produced meals and wondering why Mommy's at home and doesn't want to spend time with her.
Lots of churches offer programs. Even if you aren't a religious sort, these church based programs aren't a bad idea. They don't get too preachy because a 3-yo won't understand it anyway. It doesn't hurt to check around and check out programs. At least you know what's out there when the time comes. You can ask them about registration dates and waiting lists, but most don't take registration more than a few months in advance, so if you really like a program, you can find out when to be the first one in line.