When singing,

If it doesn't sound funny on recordings with the mic either
A. placed higher up closer to the nose
B. placed far away

And C. doesn't leave you with a sensation of a resonance ceiling on your high notes (like your jaw and tongue is fully dropped on an ah but your throat still can't create enough vertical space to get you higher)

Then you are fine.

But if any of those problems above are happening, then you should try to reduce it which is honestly very hard to explain how to do. The action is very simple - closing up the velopharyngeal ? port, very similar to raising the soft palate. But that's not an action we can just control at will just by thinking of that exactly in isolation (nothing in life teaches us exactly how), you often need to think of a weird reference or combination of a few (I think that's what I do) to achieve it indirectly.

Felipe once listed for me a whole bunch of references/sensations for raising the soft palate to eliminate nasality, maybe he could chime in here.

Taking a more top-down approach to your warm up (focusing a lot on the A4+ area) can also help because high notes tend to require a more lifted soft palate and if you get that warmed up just using vowels only, it will tend to stay throughout your range easier and prevent nasalization on the vowels.

As for speaking, I might have nasality in my speech too, not sure. But I do believe it decreases when I'm warmed up. As I said I've developed a degree of control over it as well, only because I used to be nasal and learned how to get rid of it.

Lastly about nasal resonance. Never really understood that term but I think it's just a poor replacement for head resonance. When in actuality head resonance doesn't reside in the nose in any way. I think someone just got confused along the way and started calling it the wrong thing. That's my theory at least.

Phil Moufarrege said
Firstly MM and NN sounds will always be nasal.

also NG.