I'm curious about this too as I have a WCBP, from what I've read it's mostly their crest that tells wether they are boys or girls. But if their crest is slicked back looking then it's most likely a roo, if it has a pouffy mushroom shaped crest it's most likely a pullet.
And I hate to say but yours looks a lil slicked back...you may have a roo. But I'm not an expert or anything lol!
Here is what mine looks like I'm fairly sure mine is a girl..but again I could be wrong!
With polish it can be really tough to determine gender. You'll probably find yourself going back and forth with it. A mohawk crest usually means male, and a round afro is a female. But again, with polish, sometimes the crest isn't always accurate. My GLP had developed a Mohawk at around 4 weeks and crowed at 8 and a half weeks.