It's hard to say what shows confidence, but body language can.
Here are some of the things I've been thinking about:
When I first saw my supervisor, I noticed something slightly different about his body language. He has a background in sales, so it makes sense that he's worked on the proper posture and body language, because being able to project confidence is important for a salesperson.
So I looked up his body language and was surprised to learn that he was using textbook confident body language. Confidence and dominance are a bit confusing to me, in terms of body language--and they often overlap when I read about it.
Here are some of the things he does to exude confidence without even saying anything:
He tilts his head back and raises his eyebrows often, while maintaining eye contact. This is a sign of dominance and is often seen in people giving orders, in fact, I do this to my son, unconsciously, when he's disobeying. Note that my supervisor doesn't give orders or have hostile facial expressions--so this just exudes confidence without seeming hostile or arrogant (the same body language can seem arrogant and probably shouldn't be, IMO, coupled with folded arms or any kind of scowl or grimace.)
He makes eye contact often, and he positions his body to face the person he is addressing.
Then, he sits upright, and the tilting of the head back naturally puffs out his chest and shoulders a bit--another confident look (without being too peacockish.)
He holds his body in a relaxed open manner.
Often, men will also sit with their legs more open, this projects some kind of alpha male quality. Women, if you notice, will often close or cross their legs. It's actually funny to watch for on a bus--you'll have a guy with his legs all spread apart, and a woman sitting next to him closed up like a clam.
Taking more space (both with the puffed out chest and with the open legs) exudes a type of dominance or confidence.
So---I think it's important to work on your own self confidence and self esteem, but I also think that in a dating situation, when you are nervous, you can try one or two body language cues to sort of fake it. It's not really the appearance that matters in the long run, but it can help during initial contact, IMO.
Being confident when you're talking, well that's different. Overall, I don't know how to be confident inside, in many situations. I can talk to strangers fine, but if I think I'm ever going to have to talk to a person again then I get totally anxious and it's really hard to be confident.