I took a flyback transformer out of an old CRT television.
This is how it looks like on the top:
And this is how it looks like on the bottom: http://i.imgur.com/8QYwPhQ.png
W1 is a thick red wire that ends with a suction cup; W2 is another red wire similar to W1, but thinner and without suction cup; P# are the pins.
In order to identify the 0 V pin of the secondary coil, I connected the + input of a voltmeter to W1, the - input of the voltmeter to the 0 V output of a power supply, and I tried touching each pin of the transformer with the 24 V output of the power supply.
I read +0.03 V on P0, +1.79 V on P4, 0.00 V on the others. However, I saw on the internet that by using this method I should read something between +5 V and +10 V on the 0 V pin of the secondary of the transformer. I couldn't get that voltage on any of the pins: am I doing something wrong? Is probably P4 the pin I'm looking for? How can I be sure? Thanks.