The thing with the finger lengths is not an absolute, but it’s a fairly strong indicator. In my high school biology class, with four girls and about a dozen boys, there was only one exception (and no, we never did let him live it down).
There are also definite differences in facial shape, but only on average, and not necessarily in individuals. Short of a DNA test, I don’t think that there’s any foolproof way to tell, but if you see a person with long index fingers, a negligible Adam’s apple, wide hips, and a rounded face, it’s a pretty good bet that that person is female.
My ring and index fingers are exactly the same length. What does that tell you?
[seinfeld]Man-hands![/seinfeld]
–Tim
Hmm . … my sister and mother are, by Rysdad’s method, both male. Good to know;)
Osip has a related question.
How does Estrogen and Testosterone afect characteristics of the human body? These may or may not effect some features to change and become more or less apparent.
Maybe this is a path we can explore to try and answer the OP.
Osip
Zoggie, I’m known here as a dull one here. I do not understand what do you want exactly to know? Do you want to know whether it is possible to tell males and females apart with 100% certainty base solely on what you see on their faces? - NO.
By their skeletons? - YES.
By their live dressed figures? - NO.
The answer could be something like: How do we …? By experience, culture, etc.
Ask if you were adopted, then tell your sister to shave. 
But it really does work…most of the time. In fact, I’ve never met anyone who didn’t have fingers of a length consistent with their gender. (People who work in tool and die shops excluded.)
Yeah, mine is really how do you assume you’re seeing a male/female by their face. Not that you’re always right…but most of the time, you can.
You answered by experience and culture- could you elaborate?
Osip- That sounds like a valid point…does anyone here know?
I’d say it means a little bit o’ both, which can be interpreted as meaning you’re a lesbian…it works! 
Q.: So how can you tell the difference between males and females, by their facial features?
The answer to this might be: you can tell the difference between males and females by their facial features most of the time because you have experience; you saw a lot of faces during your life. A three year old child will make more mistakes than you. By culture I meant that one is a better expert in his town than in unfamiliar milieu, where peoples’ faces are different.
You are more right than you know…
Before puberty: You really can’t. The faces are virtually identical prior to puberty. How one dresses and does their hair has a lot more to do with how a person looks when they’re a kid. When older, guys develop secondary sexual characteristics. These include facial hair, an elongated and squarer jawline, and a heightened ridgeline above the eyes. We notice these changes and can tell whether it’s a guy or not. Women don’t go through these facial changes…their faces stay very much the same throughout.
Jman
Mine are the same length on my left hand, and on my right hand my ring finger is maybe 3 mm shorter than my index finger. So what does this mean?
FWIW, I seem to recall that this little tidbit is inconsistent with respect to lesbians, although I forget how.
Here’s the link to the previous discussion about finger lengths and gender/sexuality. The Dopers who posted did not seem to fit the classifications set out by the original study.
http://boards.straightdope.com/sdmb/showthread.php?threadid=22842
Okay, another anatomical one nobody has mentioned yet. I remember having a class in which we checked this out, and it seemed to hold as well:
Hold your arm straight out in front of you, palm up. If you are male, the upper arm and forearm will likely be straight, or produce a very slight angle. Most women have a noticeable angle formed at the elbow, moderate to sometimes quite pronounced, as well as being able to bend the elbow backwards quite a bit more often.
OK, Anthracite, care to report your findings on this one?
Which direction is the angle supposed to go now? I’m having trouble visualizing it.
Another thing: if I press my hand down on a flat surface, palm down, I can bend my fingers up by themselves (that is, without using the other hand) to at least a 30-40 degree angle with the surface. I’ve been told that is another female trait…
And what about Mini Driver? Talk about a wide, chizled jaw line…<shudder>
I’m female and there’s little or no angle…
I can do that, though … hmmmm. Actually, I can do my left hand distinctly better than my right, directly attributable to guitar playing (I’m right handed, but practically every finger flexibility trick in existence works better with my left because of this).
I rememember the arm thing being shown to us in a bio or health class, about like Chronos remembers the finger thing. It did seem to hold pretty well in our small sample. Again, not an absolute, and I can’t say that I’ve personally investigated this in the last couple decades.
Anthracite - to visualize the angle - holding your arm out in front of you with the palm and inside of elbow facing upward, hold the arm straight in front of you, pointed downwards at about a 45 degree angle, locking your elbow. Look down your arm at the angle made by a line drawn along the biceps of your upper arm and one drawn along the underside of your forearm. I’m talking about an angle to the side, not about an angle in the plane you are flexing the arm in.
Zoggie, you’re just wierd, then … I hope I don’t need a smily there - I hate using emoticons.
Men get hair in their ears, women do not. Im not sure about hair in the nose. But a nice full beard would probably indicate a male…
In another of my date losing ideas, men have larger brains then women, so a larger forehead is male.