Hmmm, I am no expert but, in my personal experience, female white faces have actually an all grey face or a bit of white with grey patches, pearls or even little lines - when the face is all white, like the bird on the picture, it's usually a male. Not that it makes any difference but males will call and call and call and call all the time during breeding season (and all year round if you don't keep them to a strict solar schedule with full exposure to dawn and dusk and on a low protein diet) if they don't have a mate while females will also call for a mate but the call is different... just two notes and very monotonous (can drive you nuts
).
When it comes to sexing tiels, you usually have to wait until they are more than 6 months old unless they are a sex-linked mutation or an albino in which case you need to have them DNAd.
One more thing, having them loose all day long is a wonderful thing but unless they live in a dedicated bird room (a room that is ONLY for birds with no human furniture or anything in them), it's dangerous. Birds should never be left loose and unattended in a human room. It's dangerous.