Get any 20c from 99-present, other than 2004.
Get your 2004 coins.
Put another year next to your 2004 coins and you will be able to see very easily that the majority of 2004 20c have a smaller head. To confirm it is the smaller head you look at the first A in Australia. If the top of the A is blunt then you have a small head.
If you cant tell the difference between your 2004 then chances are they are all the small head variety.
I will put a photo up in a little bit.