Both sexes have 3 rows of "peas". The difference is that cockerels get their full width much sooner than the pullets. So, it's more a matter of timing and development rate than pullets only having a center ridge (which they usually do till after 12-14 weeks old or so.)
By 13-14 weeks, males already are showing stringy, shiny saddle feathers, so that would be how you'd tell at that age.
By 13-14 weeks, males already are showing stringy, shiny saddle feathers, so that would be how you'd tell at that age.