“How do you know that you have received a word of knowledge?” The Bible say's that the Holy Spirit will bear “witness” with our human spirits.

This inner witness is what I call an "inner knowing." Suddenly, you will receive a “without-a-doubt type of knowing” that He is trying to transmit to you. It can come in the form of a thought or an impression on your mind, or possibly it can come in the form of a vision or a picture in your spirit.