Dispite all the so-called experts who have written rubbish all over the internet, there is no way to sex canaries for sure at this time of the year.
Soon the males which are in breeding condition can sometimes (and only sometimes) be sexed by viewing the cloaca which looks like a swollen pimple.
If you have an good eye for tints, some cocks have a slightly deeper colour yellow, orange or green but you have to see them side by side to notice the difference. White ones are of course unsexable by colour.
The only sure way to sex them is by the cock's singing. Some hens, especially when kept separate, will try to 'sing' but never anywhere near as good as a boy.