There are multiple factors that help me in sexing a chick, both when they are young and when they have grown to the average sexing age of six weeks. Note that there are slow-maturing breeds that you can only sex at approximately sixteen weeks old.
This chart below is a good reference for...
I have only studied chickens for one year, but I have had them for six years. It didn't take long at all for me to learn how to sex chickens-it was one of the easiest things I have learned about! It does take time, but it's easy, and you can learn it fast once you have the knowledge.
I thought you meant real play-fighting at first, which birds definitely don't do. Thanks for clearing that up!
The first three that I listed as cockerels are very suspicious, with their large comb growth for such a young age. Let's hope that they are only fast-maturing pullets that look like...