I have used this test for the last few years and it has proven itself to be pretty reliable. I had two of my freshman runers do it today and I decided to post it as an event so as to keep track of my kids' results.
Check this site for an explanation of the test and the calculator that is used to determine the potential 800m and 1500m times.
http://www.brianmac.co.uk/kosmin.htm