How do you know it's a singing female and not a male?

[Maleficum]

My new canary (the only surviving from the rescue attempt. From the same place the canary I wrote about in this post http://www.finchforum.com/viewtopic.php?f=74&t=35611 ) is supposed to be a male and he sings. But now I have started wondering if he really is a he or if he is a she. He (Baltazar) sings but he sings really quiet and not as intensive as my other canary (Malte). Malte and Baltazar get along really well and I have seen Malte feeding Baltazar on a few occasions.
Right now they are moulting and Malte almost never sing right now (he sang a LOT and and very intensive during the summer) but Baltazar still sings now and then (he sings the same as he did during the summer).

I tried to catch him singing on a video but Sixten really likes it when the canaries sing and also wants to make sounds when they do so maybe it's to hard to really hear Baltazar singing.
https://youtu.be/BlK4zIJ9AjA

Is he a he as he's supposed to be or is he a singing she?

[Sheather]

If one feeds the other it's almost a sure thing that Balthazar is a female, some sing well, but as you say, never as loudly as a male.
All of the hens I've kept sing to an extent, some better than others.

[Maleficum]

Sheather

I manage to get a new film of Baltazar singing https://youtu.be/lLvAiS6KEs4 (this time only with my neighbours dog trying to get my bunnies attention).

Dose males never feed each other? (I have more experience with birds that don't care to much of the sex of there partner like budgies and ringneck doves).

I suppose I have a better chance of being certain of Baltazars sex in the spring, see if he start laying eggs if he's not to old.

[Sheather]

From the softness it sounds like a female song to me.
To my knowledge male canaries will never feed other males, usually males are aggressive with each other during the breeding season.