You have to practise it to know it... and practice isn't the key
word.
Relaxing the body is very important. If your body isn't relaxed, no
point practising it...
I don't know what's called powerful. I can only describe what I
feel.
At the basic level (about a few months after you start), you will
be able to feel energy flowing. If someone of the opposite sex is
near you, he/she may complain of some weird feelings. Also, at this
level, your hands will get warm during practising.
Over the years, when there is "enough" energy, you must be very
careful when hitting people. Even a friendly pat. Once I
accidentally pat too hard on someone... the person jumped up in
pain. 
Even more powerful ones... got to find master le...
