In order to access our true strength is is necessary to allow energy in. (In contrast to this, when we are angry, we push our energy out). When you are in a situation, open yourself to what is going on. Allow the energy of the situation in so you can feel what is going on. This allows your intuition or knowing self to kick in. Then you trust your heart or intuition (or gut) to tell what you to do and take action- even if it does not make sense intellectually. Acting from from your heart is always strong and true.
The best indicator of true strength is that it is always born of gentleness. What we do may be be firm and really strong with people, but it is always based on gentleness and compassion. This is because it came from your heart. It is based on the energy of the situation and the empathetic response of our intuition. Carl Jung called this acting based on the "requiredness of the situation."
So if your strength or strong action is gentle and compassionate, it is true. If it is not gentle, it is something else- usually control or anger. It is like that Keb Mo song, "I don't know what it is, but that's not love." Or in this case... that's not strength.