Hi,
The normal japanese and Italian (also known as golden) are easy to sex. The male has a chestnut coloured plain breast, and the female has a speckled breast.
If you have british range, either with white tuxedo or not, you will need to "vent"sex them. Hold the bird with the breast facing up, and just above the tale is the vent. Males vents are more dome shaped, but this is sometimes hard to tell by. Give the vent a gentle squeeze between your thumb and forefinger, and if some foam is produced, then it is a male. If they have just mated though, you may get foam from the females vent if you know what i mean.
Hope that helps,
Cheryl