Good morning, @Mattybaby0331 ...
Unfortunately, I do not know of any way that you'd be able to tell when an instructor created and/or published an assignment in your course to make it available to you and your classmates. To be honest, I'm not even sure if there would be any way to do this programmatically (this would be outside of my realm) from a Canvas administrator's position. You may be able to try contacting the Canvas Help Desk, and they may be able to give you more information on that assignment, but I'm not 100% sure on that, either. How do I contact Canvas Support?
Sorry that I don't have better news for you, but I hope this has helped to answer your question. Take care...be well.