Not using an instrument may not be the best way to proceed -- there are two competing forms of maturity models.
In the typical stair-step model (e.g. Kernzer's Project Management Maturity Model) there are typically 5 steps in any maturity model and they typically parallel the SEI maturity model. To reach Step 1, you should have standard definitions; Step 3 which is typically a good point to aspire to is having repeatable processes. So I would suppose that a rough estimation is deteremined if there is an emerging project management methodology that is being repeated. That should be rated 3 and above.
PMI's OPM3 (Organizational Project Management Maturity Model) doesn't believe in the stairstep approach. It measures several different dimensions of maturity, allows you to plot them visually in a spider diagram and to compare your progress with similar sized companies. Rather than focus on getting to that "next step", it encourages companies to continuously improve their processes in multiple dimensions and make decisions about what to do next.
So I mentioned not to skip the assessment instrument ... this is important because this is not purely scientific. In addition, the benefit of the instruments is that you know where you are, the outcome of administering the assessment is typically an actionable list of steps to take or choose from to improve.