Ooh, a full day for a 2.5 year old is tough...though are you looking for a 3yo program? You may have better luck with that though I can't think of many preschools that do a full day program for 3 year olds. I can think of ones that are affiliated with day care programs. You may have narrowed your search already so your market may not be too big if that full day MWF is a firm criteria. My ds' 3yo program was T, H, F with an option to stay all day T and H but not Friday.
I personally just took all referals and visited every school that sounded good. I got pretty good at weeding out schools quickly.
One of my keys was to find out about their potty training philosophy. When I was in college I took an early childhood development class taught by the director of the university's childcare/ preschool facility. She said that schools should not require potty training by X day. She said that schools that require that may not be practicing a "developmentally appropriate" philosophy. She said that the school should respect the individual needs of a child and not assume that all kids can be ready to be pt'ed by a certain age. So, that automatically weeded out a chunk of schools. The ones that were left were all very good and talked at length about supporting the individual child and were developementally oriented. (This wound up being a very good thing for me to look for b/c my ds was very stubborn. He pt'ed 2 months before he turned 4. He did it in one day- when he was ready. I am so glad that there was no pressure to rush him. I tried that and it was a nightmare.)
I also wanted a school that was "play based" I believe that kids learn best through play. So, one school was out when they said they had their 3 year olds start the day with a worksheet. I didn't see any benefit to that.
I really just wanted a school that would work with my son and didn't want him to fit into a mold. I wanted them to nurture what he had to offer and help him learn to get along well with others and be enthusiastic about learning.
There were some great choices for us and our budget. There were schools located at a large university where kids would take daily field trips to the campus and surronding parks and museums. There was one that had a swimming pool and computer lab. We wound up choosing a pretty simple one because it just felt like a good fit for us.
I think the guidelines are good from the NAEYC but you just have to get out there, take tours and collect info.
If I think of more I'll let you know. I was pretty into choosing a preschool 2 years ago and did a lot of research. :)