welsummer pullets have a very defined V shaped brown marking on their heads, while the roos may have the same V shape, it is much more pale and less defined. The pullets also have brown around the eyes (looks like eyeliner) The roos may have this also, but again much less defined. It helps to have several chicks so that you can see them in comparison to each other. I've been told that this is not always 100% accurate, however in my experience it is.
As for the brooder, what is the paper bag for? I would try to postition the heat source from above. The little buggers will be up on that bulb way sooner than you'd think. As quick as 2 weeks maybe. It is hard to tell for sure, but it look like it would like a little more ventilation. Is it a display case of some kind? Cool idea if it is.
It looks more than big enough for 3 chicks. In fact you may need to divide it with cardboard or something so the heat won't travel away so far, three chicks won't be able to huddle much if they are cold.