Conditional Variables in Scriptable Objects

The Editor-friendly path is a "custom inspector". In Unity API terms, this means extending the Editor class.

Here is a working example, but the doc link above will walk you through a lot of the details and additional options:

using UnityEngine;
using UnityEditor;

[CustomEditor(typeof(Test))]
public class TestEditor : Editor
{
    private Test targetObject;

    void OnEnable()
    {
        targetObject = (Test) this.target;
    }

    // Implement this function to make a custom inspector.
    public override void OnInspectorGUI()
    {
        // Use the editor auto-layout system to make your life easy
        EditorGUILayout.BeginVertical();
        targetObject.testBool = EditorGUILayout.Toggle("Bool", targetObject.testBool);

        // GUI.enabled enables or disables all controls until it is called again
        GUI.enabled = targetObject.testBool;
        targetObject.testString = EditorGUILayout.TextField("String", targetObject.testString);

        // Re-enable further controls
        GUI.enabled = true;

        targetObject.testInt = EditorGUILayout.IntField("Int", targetObject.testInt);

        EditorGUILayout.EndVertical();

        // Mark the object dirty so it's saved to disk
        EditorUtility.SetDirty(target);
    }
}

Keep in mind that this script uses Editor-only APIs, so it must be placed in a folder named Editor. The above code will turn your inspector into the following:

That should get you rolling until you are more comfortable with Editor scripting.