I don't think there is any way to prove you have tinnitus. The ENT will likely only be able to tell you that you do not have a physiological cause. If I remember correctly, there is a rare condition where the one of the small bones in the ear can break and can cause a tinnitus like condition. Somebody here on NT had such a condition.

Would your boss allow you to have an MP3 player ? You can use one with on the ear headphones that allow you to hear people talking around you compared to ear buds that block most other sounds.

So, that would be the question to ask. Is your tinnitus bilateral ? If it is, the bone issues would not be likely. If you have tinnitus in both ears, even if it is louder in one, that suggest a neurological condition or problem with the auditory nerve system. So, the question would be: "I understand there is a rare condition where a little bone breaks or become dislodged causing tinnitus like sounds. Is this a possibility with me ?"

An audiologist/hearing aid specialist may be able to do some tests that show some specifics about your tinnitus. I know my tinnitus is just above the key of C#6 on a piano, just above 1108 Hz. Maybe an audiologist can check the frequency multiple times to confirm you can repeatedly identify the frequency of your tinnitus. That may be what some do with tuning forks. Maybe the ENT has a frequency generator to do this.

My best to you.