Generally it takes a long time to develop this skill, at least it did for me. I have been working with energy for over 10 years now and find that energy balls are an excellent way to clear out stagnant energy, or deter a negative entity from staying in your space.
Once purely for fun and to see if others could sense anything when I do it. (trying to prove to myself that it was not purely in my mind) I shot an energy ball of white light at an unsuspecting friend.
We were standing in line waiting for someone, I would never do it to someone who was busy doing something that required their attention, or would not have approved in the first place. Fortunately for me, my friend is very open to this sort of thing, and also has a good sense of humour.
Anyway, back to my story...When it hit her shoulder she actually stumbled, and then looked straight at me and asked me what I did. I was really impressed that someone else who didn't know anything about energy balls could feel what I did. Which once again confirms to me how much we don't actually know about our universe. :)
I have had other people be able to see when I work with energy, but as for the general public being able to see it, I would like to believe that that is possible, although I have never heard of it happening.
I don't think that a psi-ball could ever stop a solid object, although I am sure there are other forms of energy that could.
Yes, it could be dangerous to throw at someone. If they have issues with their chi energy you could throw that off. Also if someone is more open to these things, you could startle them. That would be the same as saying boo for example. Ok for fun but not when they are driving etc.
In order to make an energy ball, cup your hands like you are holding a snowball. Imagine pure white light flowing out of your palm chakras. Watch as it pools between your hands. With practice you will begin to feel the air between your hands change ajnd become more warm and dense. Once it feels more solid, swiftly through it at the wall, and sense out the difference between before and after. If you have an open minded friend you could practice throwing them at each other and try to sense the difference.
Don't be discouraged if you aren't successful right away. It takes a lot of concentration and practice.
Good luck!
:)