Vector3.Angle

Parameters

Returns

float The angle in degrees between the two vectors.

Description

Calculates the angle between two vectors.

The angle returned is the angle of rotation from the first vector to the second, when treating these two vector inputs as directions.
Note: The angle returned will always be between 0 and 180 degrees, because the method returns the smallest angle between the vectors. That is, it will never return a reflex angle.

using UnityEngine;

public class AngleExample : MonoBehaviour
{
    public Transform target;

    // prints "close" if the z-axis of this transform looks
    // almost towards the target

    void Update()
    {
        Vector3 targetDir = target.position - transform.position;
        float angle = Vector3.Angle(targetDir, transform.forward);

        if (angle < 5.0f)
            print("Close");
    }
}

Additional resources: SignedAngle function.