I asked the same thing re week old Mandarins and the response was to vent sex or wait....sometimes leg color and bill color tell us too- but that takes waiting or heaps of experience too. I'll try vent sexing one of these days....they are so jumpy every time I go out to just talk to mine!
You can have a vet take blood and do a DNA test to sex them, it costs about $25 per bird plus whatever cost the vet charges. My thought is how important is it for you to know the sex at this age? It might be worth the money for you to have it done if there is a specific reason.