The first criteria is whether one can "feel" the existence of chi and the second criteria is whether one can "feel" movement (or sometimes a potentiality of movement) of chi. If non of these criteria is met, one is not yet truly doing one's practice. Period. The reason is that it is all about internal "feeling", it is either there or not there; no amount of argument or logical reasoning can save one from a position of not being there, if truly one is not there in the first place! Corollary 1: if you don't know what I'm talking about, there is a high chance that you're not there [another possibility is the failure of my communication skill, which I admit will betray me from time to time]. Corollary 2: if you start having such "feeling" recently, don't feel too overjoyed yet; chances are that you've just "entered the door" (入門).
In tai-chi, it is called "listening" (聼勁). Since tai-chi is a moving form, one has to be able to "feel" or can "listen" one's chi flow during movement. It is an advance form of listening, the basic form is listening during stationery form Nei Gung, like zhan zhuang.
What is the purpose of activating and moving chi during tai-chi? Chi is primarily used to build up, align, and strengthen one's physical structure, with the objective of making a practitioner's body "ready for combat". In view of this objective, a sub-yardstick of chi-movement in tai-chi will be the opening and strengthening of one's joints (including ligament, connecting bones and tendons). Joint-training is fundamental to structural integrity both for human body and for man-made mechanical structures like a common crane. A weak structure is not suitable for combat nor can it move heavy objects around.
Therefore, a tai-chi practitioner (assuming he is truly doing tai-chi) will be able to "feel" or "listen" to chi (or jing, which is the same thing) doing the the internal task of aligning his body structure and lengthening and strengthening his muscles/tendons/ligaments/bones. Sometimes the power of such chi/jing when activated can be very strong - yet, or actually as an essential requirement for such activation, one mind's will be super relaxed and focused! This is nei-gung par excellence!
I shall talk about its implication for Zen/Taoist meditation in some future posts.