Calculate rotation delta

I need to get the rotation delta of an object.

Let's assume you have these few lines of code:

Vector3 previousForward = transform.forward;
transform.RotateAround(someRotationAxis, someRotationVector * Time.deltaTime);

Quaternion rotationDelta = Quaternion.FromToRotation(previousForward, transform.forward);

Now this nearly works but depending on the rotations direction I get values of 0 + X or values of 360 - X.

What did I miss?

Where are you getting those values from? If you're reading the .x value of the rotationDelta Quaternion, you shouldn't - it's probably not what you think it is.

How about using Vector3.Angle instead?

float angleDelta = Vector3.Angle( previousForward, transform.forward );

Although - in the example you give above it seems as though there's no need to compute it at all, since you're stating the angle you want to rotate by in the preceding line!