Head shaking but vet said no ear infection

My cat shakes his head a lot. Last time I took him to the vet I told him to check his ears, which he said were fine. If that's the case, why is he shaking his head?
Is there any way for me to check his ears myself?

I look in my cats' ears all the time and usually clean them out once a month or so. My one cat has always had a lot of gungy build-up in his ears and he will scratch if they get too dirty. The other cat has scratched his ears until he cut himself!
But they don't have mites (vet check them out every year) and they don't go around shaking their heads.

How is your cat's balance? Does he walk and jump normally? I'm wondering if there's something else going on and it's not an ear problem.

His balance seems fine. I read about a neurological problem that causes a different type of head shaking, like a bobble head. That's not what he does.
However, I am concerned. He's due for a shot soon. I guess I'll have to push the issie when I take him for it.