[Nail]

If the point is on the "wall" TR_GetPlaneNormal returns normal vector to the collison plane of a trace
If the point is in free space TR_GetPlaneNormal returns normal[0] = 0.0 normal[1] = 0.0 normal[2] = 0.0
How do I know managed to get TR_GetPlaneNormal normal vector or not?

0 0 0 can also be the correct result

[blodia]

come on, you answered yourself. use TR_DidHit to see if the trace collided with something, if it did then you would get the correct result.

[Nail]

In some cases TR_DidHit returns wrong results.
For example, ZM_Christmas_city skybox

[blodia]

well depending on what flags you use it may not be the wrong value, traces will collide with anything whether you can see it or not. even if it does you can still use the other trace natives like TR_GetFraction, TR_GetEndPosition with TR_GetPointContents, TR_GetEntityIndex etc. there are many ways to check if you hit something.