Any sound you hear in the absence of actual sound is by definition tinnitus, as far as I understand. Once would expect an absence of sound to sound like nothing, no?
I don't know. I've never heard anything different from what I hear now in the absence of sound, nor have I thought to question the normalcy of what I hear until now. Without a reference point, I cannot make a comparison.
Been strict you can't know for sure. Birth deaf people probably suffer strong tinnitus for their lifes without realizing. Oliver Sacks has nice essays extending on this.
But if tinnitus comes during your lifetime you do have a reference point, and you will most likely notice.