Hens are generally larger than roos, but not enough (in my experience) that I would use that to tell them apart.
If they're Pharaoh (or natural) in color, then once they get to be three or four weeks old (once they're fully feathered), you can tell them apart by their breast. Hens are speckled and roos have kind of a rusty color. With some of the other colors (like white obviously), you can't sex them that way.
There's a way to vent sex them if they're mature and they're the wrong color to sex them by looking at them. It would be easier if you watched a video of that than trying to explain it, but if you go to YouTube and search on "vent sex quail" several videos will come up.
Finally, if you have more than four or five birds, it's highly likely that you have at least one of each. If you have five quail, for instance, the odds that they are either all hens or all roos is only about 6%. UNLESS someone already separated them according to sex (like if whoever you got them from was keeping only hens for eggs), but I would think they'd have told you if that was the case.