it might be a bit early yet for preflowers but if you have at least a 10x magnifying glass you might be able to spot some (look 2-4 nodes down from the tops or )...they're gonna be especially hard to see in a pic this soon. i found the 1st one today after a long search through the patch and yes, it's a grrrl!

it's too late now but next year you might want to check into topping/pinching them while they're in seedling stage...but i definitely would not do it this late if they were mine.