White plymouth rocks, boys or girls?

[alevay]

Here are some pics of my flock. They are all white plymouth rocks that I have had for 8 weeks now. I got them from tractor supply. They are a straight run. I am trying to figure out how to tell boys from girls. Some have combs, others don't. How do you tell?

 

[iwiw60]

Third pic down looks like a roo to me. Fourth pic down looks like a hen...that's about all I can tell.