I am now the proud owner of 14 juvenile finches!!

I am trying to sex them, but I keep reading here about molting into adult colors etc. So, my question is,,,How do you know when a finch is molting? I notice over time they look different, but don't notice the feather changes or pin feathers really.

My Star, RCCB and GS all seem to have the coloring of females still, but two of them are singing, so I know they are males. The other two, I don't know if I have not heard singing or if they are indeed females. I wonder if these species have a first and a secondary age for molting? When can I expect to see their final feathers? Thanks.