How can I check if I pressed F in the class other than Update()? C#

My question maybe is simple, but I have problem with this. I want to my script will send a message to GUIText called showText, which shows text on HUD. My problem is that: When I enter the trigger without pressed F (there is only "Use", which has F key applied to Input Manager), the message is right, but when I press it, the message is still the same. To show my expected message, I must exit the trigger and enter it again, but with hold down F key. This is I want to fix. How can I fix my script to show the other message when I press F key? There's my full code of DoorZone which uses somes variables in other script

 using UnityEngine;
 using System.Collections;
 
 public class DoorZone : MonoBehaviour {
     
     public bool doorIsOpenAlready = false;
     public int pointsRequired = 750;
     public GUIText textHints;
     
     bool UsePressed = false;
 
     void Start () {
     
     }
     
 
     void Update () {
         if(Input.GetButton ("Use")){
             UsePressed = true;
             //Debug.Log ("F is pressed!");
         }else{
             UsePressed= false;
             //Debug.Log ("F is not pressed!");
         }
     }
     
     void OnTriggerEnter(Collider col){
         if(col.gameObject.tag == "Player" && !doorIsOpenAlready){
             
             textHints.SendMessage ("ShowHint", "Would you like to open this door for "+pointsRequired+" points?");
             
             if(UsePressed){
             
                 if(Inventory.points >= pointsRequired){
                     textHints.SendMessage ("TurnOffText");
                     transform.animation.Play ("doorOpen");
                     doorIsOpenAlready=true;
                     Inventory.points = Inventory.points - pointsRequired;
                     Debug.Log(Inventory.points);
                 }else{
                     textHints.SendMessage ("ShowHint", "You haven't got enough money!");    
                 }
             }
         }
         
     }
     
     void OnTriggerExit(Collider col){
         textHints.SendMessage ("TurnOffText");    
     }
 }