"How do I know whether I feel" is not a sensible question. The only person qualified to know what you feel is yourself.
At any rate, for singing you want to project your voice. You are not interested in getting your neck or pharynx to vibrate but rather the air in the ear of the listener.
If you are singing efficiently, your vocal cords are working against reasonable mechanical and acoustical resistance and can transfer a good amount of energy to the air with comparatively small movement/vibration themselves. This resistance comes from resonators provided by good posture, breath support, an unconstricted throat singing into the resonances of your head cavities and the room behind it. When doing that, it is usual to feel the resulting vibrations in the "mask".
So to come back to your original question: when you don't feel vibrations, you know that you don't feel vibrations. It's as simple as that. You can either change your sensitivities or the sensations or both. Once your sensitivity manages to connect with sensations, either by different attention or by different vocal production, you can use that feedback for improving your actions.