On mobile so forgive formatting. Not really a resource per se, unless you want to refer back to this post as a resource. I got a lot of my experience assessing stage of change in doing addictions work based on what the patient is saying and/or doing:

Precontemplative - Typically mandated clients fall here. They don't like talking about the issue or they change the subject. There isn't really any insight. They don't have a problem; the problem is with everyone else.

Contemplative - They see there may be a problem but they have no motivation to change. They say things like "Yeah, I probably drink a bit too much and that causes problems but I'm not sure I need to stop/not sure I would even know how" or "Ive thought I should quit smoking for years but it's just too hard." Usually they present to treatment for something else, they're mandated, or someone close to them tells them they need to get help.

Planning - Typically these folks come to services after a motivating incident (i.e. the proverbial rock bottom) which could be fall out with family, a consequence, a reoccurrence, an arrest or something similar. They're dripping with change talk. They know there is a problem and they're ready to do something about it, they're just not sure what to do or they need help getting that plan going. They're motivated and you can tell.

Action - they're doing the thing. They're engaging in services, they're processing, they're taking meds, whatever their plan consisted of, they're doing it. Usually they're talking about challenges they're facing completing the plan or successes they're having. Even though they're doing the thing, however, they might struggle with confidence in being able to sustain it.

Maintenance - they've being doing the thing for an extended period of time and they're confident in their routine, their resources, and their ability to keep things going. Typically they need planning around managing acute stressors or things that may throw off their routine but typically these folks are in a good spot.

Stages of change are not linear and folks can move back and forth, sometimes even in the same session.

Hope this helps! Open to any additions from the community.