For the form student and from a good site :

History over the last 17 years shows that the Melbourne Cup winner is:

1. Is highly likely to start at less than 20/1 in bookmaker markets

2. Will be coming off a top last start performance, either winning or finishing very close to the winner.

3. Has time ratings [110 or better] in one of their last two starts that demonstrate the ability to run fast overall time.

4. Can be drawn wide.

These simple rules should narrow it down to 3 - 5 good prospects.