Use this InspectorReadOnly attribute for public fields that you want to easily inspect in the editor without allowing modifications. InspectorReadOnlyDrawer needs to be inside an Editor folder.

InspectorReadOnly.cs

using System;
using UnityEngine;

// Usage: 
//
// [InspectorReadOnly] 
// public float someValue = 5f;
//
public class InspectorReadOnly : PropertyAttribute {

}

InspectorReadOnlyDrawer.cs

using System;
using UnityEngine;
using UnityEditor;

[CustomPropertyDrawer(typeof(InspectorReadOnly))]
public class InspectorReadOnlyDrawer : PropertyDrawer {
	
	override public float GetPropertyHeight(SerializedProperty property, GUIContent label){
		return EditorGUI.GetPropertyHeight(property, label, true);
	}

	override public void OnGUI(Rect position, SerializedProperty property, GUIContent label){
		GUI.enabled = false;
		EditorGUI.PropertyField(position, property, label, true);
		GUI.enabled = true;
	}

}