gistfile1.txt

/*
   Created by: Juan Sebastian Munoz Arango
   naruse@gmail.com
   2014
   www.pencilsquaregames.com
   
   Feel free to use it for your personal / commercial projects.
   a mention of the source would be nice but if you dont feel like 
   its alright.
   
   NOT for resale this on the asset store tho but can be part of your package
   without any issue.
   
*/

using UnityEngine;
using System.Collections;

public class FOV2D : MonoBehaviour {

    [Range(0.1f, 10f)]
    public float radius = 1;

    [Range(1.0f, 360f)]
    public int fov = 90;//90 degrees

    public Vector2 direction = Vector2.up;

    //used to test the FOV
    public Transform testPoint;

    private Vector2 leftLineFOV;
    private Vector2 rightLineFOV;

    void Update() {
        if(testPoint != null) {
            rightLineFOV = RotatePointAroundTransform(direction.normalized*radius, -fov/2);
            leftLineFOV = RotatePointAroundTransform(direction.normalized*radius, fov/2);
            Debug.Log(InsideFOV(new Vector2(testPoint.position.x, testPoint.position.y)));
        }
    }

    public bool InsideFOV(Vector2 playerPos) {
        float squaredDistance = ((playerPos.x - transform.position.x)*(playerPos.x - transform.position.x)) + ((playerPos.y-transform.position.y)*(playerPos.y-transform.position.y));
        Debug.Log(squaredDistance);
        if(radius * radius >= squaredDistance) {
            float signLeftLine = (leftLineFOV.x) * (playerPos.y - transform.position.y) - (leftLineFOV.y) * (playerPos.x-transform.position.x);
            float signRightLine = (rightLineFOV.x) * (playerPos.y - transform.position.y) - (rightLineFOV.y) * (playerPos.x-transform.position.x);
            if(fov <= 180) {
                Debug.Log(signLeftLine + " " + signRightLine);
                if(signLeftLine <= 0 && signRightLine >= 0)
                    return true;
            } else {
                if(!(signLeftLine >= 0 && signRightLine <= 0))
                    return true;
            }
        }
        return false;
    }

    //Rotate point (px, py) around point (ox, oy) by angle theta you'll get:
    //p'x = cos(theta) * (px-ox) - sin(theta) * (py-oy) + ox
    //p'y = sin(theta) * (px-ox) + cos(theta) * (py-oy) + oy
    private Vector2 RotatePointAroundTransform(Vector2 p, float angles) {
        return new Vector2(Mathf.Cos((angles)  * Mathf.Deg2Rad) * (p.x) - Mathf.Sin((angles) * Mathf.Deg2Rad) * (p.y),
                           Mathf.Sin((angles)  * Mathf.Deg2Rad) * (p.x) + Mathf.Cos((angles) * Mathf.Deg2Rad) * (p.y));
    }

    void OnDrawGizmos() {
        Gizmos.color = Color.green;
        Gizmos.DrawRay(transform.position, direction.normalized*radius);

        rightLineFOV = RotatePointAroundTransform(direction.normalized*radius, -fov/2);
        leftLineFOV = RotatePointAroundTransform(direction.normalized*radius, fov/2);

        Gizmos.color = Color.yellow;
        Gizmos.DrawRay(transform.position, rightLineFOV);
        Gizmos.DrawRay(transform.position, leftLineFOV);

        Vector2 p = rightLineFOV;
        for(int i = 1; i <= 20; i++) {
            float step = fov/20;
            Vector2 p1 = RotatePointAroundTransform(direction.normalized*radius, -fov/2 + step*(i));
            Gizmos.DrawRay(new Vector2(transform.position.x, transform.position.y) + p, p1-p);
            p = p1;
        }
        Gizmos.DrawRay(new Vector2(transform.position.x, transform.position.y) + p, leftLineFOV - p);
    }
}

Hey buddy, great script. I forked it and made it compatible with 3d use. Thanks.