Signs of a damaged camera mirror?

Ok I cleaned my dslr mirror with a special pen provided by nikon which supposedly is suppose to clean my mirror properly. However, my friends told me that I must NEVER touch the mirror. So I went home to check out my camera and looked through the viewfinder and luckily, didn't see any scratches or smudges. However i'm afraid I have damaged my mirror somehow. Can someone tell me what are ways and the signs and symptoms I can use to identify damage on my dslr mirror

There's no real need to clean your mirror. If it gets dust on it, it will not affect your photographs. It's annoying, yes. But as you have learned, needlessly cleaning it leads to anxiety. If your shots are still coming out and they're exposed as you expect them to be then you did no harm.

If Nikon says it won't harm the mirror, then it shouldn't harm the mirror as would happen if you touched it with your fingers, etc. That would be the case if the brush were softer than the surface material that Nikon used which might be harder than some other brands.
More to the point, you should not be exposing the mirror to anything that needs cleaning it and even if you did scratch it somewhat, it would not affect your pictures because the mirror is not in the path of picture taking light, only in the viewing.