How do you judge a school? To start you can look at the test scores. These may come in the form of the School Progress Reports (the DOE's flawed number crunching), the more nuanced but not fool-proof Quality Reviews or the number rankings given by the national websites. The black and white reality of a simple number or letter ranking is that it predicts how well your child will be able to complete a standardized test, not the quality of their education. The scores are often clung to as a measure of quality by prospective parents, but the tests are scorned by parents whose children already attend school. Assessments need to be made and there has to some kind of accountability, but the richest learning doesn't happen within test prep and the score can be a smoke screen.
You can read reviews on insideschools, study the school's website and get individual parent comments on the list serves. You can tour the schools and stand outside at 2:45 for dismissal and eyeball the parents picking up their kids. All of these are pieces of the puzzle, but none of them will give you the the kind of information about the right school for your child except the school tour that you take with your own eyes.
If you are an informed consumer, armed with the test scores and comments by others, the tour should seal the deal or expose the school as a bad fit and be the biggest factor when deciding. Even though many parents feel that there is safety in numbers (the most popular schools must be the "best") there are many quality programs laboring in obscurity (often with smaller class sizes). If you can tour several schools, you will start to see similarities and differences. Many of the differences will be comparing apples and oranges, an old building compared to a newer building, a variety of enrichment classes compared to a fabulous afterschool, etc.
Keep your mind open. Look for exciting classrooms instead of the highest test scores. Does what you see in the classroom make sense with the school's scores. Do they seem realistic? Is the work on the bulletin board suspiciously flawless or all the same? Remember to trust your instincts. There are no guarantees in life but it is very possible to get a wonderful public education in the NYC public schools.