How to see if an object is close to player?

I am making RTS game (beginner project), and in game I have workers that you can select and move, but I want to implement monsters (like spider) and if that spider is near 500 pixels to worker, it will attack, but how to program that in C#?
Sorry for my english.

 

Investigate trigger colliders and OnTriggerEnter.

 

This could help also:

Code (CSharp):

1. Vector3 PlayerPosition = Player.transform.position;
2. Vector3 EnemyPosition = Enemy.transform.position;
3. Vector3 PlayerDistanceToEnemy = Vector3.Distance(PlayerPosition,EnemyPosition);
4. if (PlayerDistanceToEnemy < 5.0f) {
5.     EnemyAttacks();
6. }