Do I have WH drakes in my mix?

My six WH ducks turned six weeks on Monday, and except for the fact that 'nobody loves me', they are doing fantastic. They are supposed to all be ducks, but I have two that I keep looking at and wonder if they may be drakes. I can't really tell by voice because they move around the yard 'en masse' and since they only love me in the 'drop the food and leave woman' kind of way I can't get close enough to individually hold and assess voices. It doesn't help that one of my silver laced Wyandottes has taken it upon herself to make sure they know where they are in the pecking order by chasing them intermittently.

In the pics they are the two with the darker heads and darker markings on their backs. I tried to get pics of them preening but if course as soon as they see me, back in the water they go! The two with the darker heads also have much darker chests as well.

Does any of this really mean anything at this age? I do know that as three day old ducklings I remember their beaks being significantly darker as well. I don't have pics from then because they were originally two of my sister's three. We ordered six, three for each of us, but she developed buyer's remorse, so I took hers.

Thanks in advance for any and all input!