GUI.GetNameOfFocusedControl

Description

Get the name of named control that has focus.

Control names are set up by using SetNextControlName. When a named control has focus, this function will return its name. If no control has focus or the focused control has no name set, an empty string will be returned instead.

using UnityEngine;
using System.Collections;

public class ExampleClass : MonoBehaviour
{
    public string login = "username";
    public string login2 = "no action here";

    void OnGUI()
    {
        GUI.SetNextControlName("user");
        login = GUI.TextField(new Rect(10, 10, 130, 20), login);

        login2 = GUI.TextField(new Rect(10, 40, 130, 20), login2);
        if (Event.current.isKey && Event.current.keyCode == KeyCode.Return && GUI.GetNameOfFocusedControl() == "user")
            Debug.Log("Login");

        if (GUI.Button(new Rect(150, 10, 50, 20), "Login"))
            Debug.Log("Login");
    }
}

Additional resources: SetNextControlName, FocusControl.