Re: How do i know when i can feel KI?
I agree with Jon regarding the most obvious sensation when feeling someone else who "uses their ki" well.
As for oneself-- even before you train it to be useful, you can feel what it is. This is my paraphrase of the coffee-cup demonstration:
Sit relaxed in a chair. Pretend a cup of your favorite beverage is on a table near you. Go ahead and get the cup for a drink... but wait, just before you actually move, abort the process. So you were JUST about to move and you stopped. You manifested the intent without the action. Well, either you can feel something when you do this or not... if so, that is ki and if not, one of us is doing something wrong!
Also, others like Saotome sensei point out that ki underlies all life processes so if you are not dead you've already felt ki.