Application.isEditor

Description

Whether the game is running inside the Unity Editor (Read Only).

Returns true if the game is being run from the Unity Editor; false if run from any deployment target.

using UnityEngine;

class Example : MonoBehaviour
{
    void Start()
    {
        if (Application.isEditor)
        {
            print("We are running this from inside of the editor!");
        }
    }
}