Silkies are notoriously difficult to sex--those, you might have to wait a long time before you can be sure. I wouldn't worry about it too much unless you start seeing hard evidence that they're boys. As far as chest bumping goes, for the most part, my girls haven't participated until a little older. However, this year, I have two 3-week-old Dorkings that are 100% girls now thanks to them growing in totally different feather colors depending on their sex, and I have seen them chest bumping with the Marans, Georgette, who I am suspecting at this point is actually just a George.
So you can't know for sure by behavior. You have to take into account everything--feather patterns, comb and wattle growth, behavior, sounds, all of it. Generally, unless they do something obvious like crow, you really can't tell until they're 6 weeks old or older. Silkies, usually 8-10 weeks or older, I think.
