http://asq.uoregon.edu/activities/

It is a link to an ASQ (Ages and Stages Questionairre). It's a short test to do with your child that tests them in 5 areas of development (communication, gross motor, fine motor, problem solving and personal-social). There is a score sheet as well that will help you score the test. If the child falls below a certain level you should try to find an agency like Early Intervetion that can help your child catch up.

I use this test all the time at work. I find it takes into account that each child develops differently by scoring on a yes, no and sometimes basis. It's also easy to follow and score and won't leave you feeling like you need a PhD to do it.

There are different months in which the ASQ should be done, 4, 6, 8 , 10, 12, etc.

Most importantly have fun doing it. Turn it into a game. Make sure your child is well rested, not sick and in a decent mood before doing it, as those factors can affect the score.

ETA: If you have a preemie... Add the number of weeks the child was early to their date of birth to get when you should test (for example: If your child was 5 weeks early and you should test on the first of January you would test on the first week of Feburary (depending on how long your months are).)