How do I know my level?

This will be from the perspective of my employer. Which has a bigger difference in terms of experience/day to day job between senior and junior engineer than other companies.

The scale

1. Junior Engineer

Completes work asigned by others with direct supervision and technical guidance. Understands subjects within their area, but has limited practical experience. Actively engaged in learning their role.

2. Engineer Follows standard techniques for focused objectives and well-defined tasks. Recieves regular feedback.

Has practical knowledge in their focus area.

Mentors others.

Understands proejcts goals and works to meet deadlines independently.

3. Lead Engineer

Adapts techniques and procedures to ssolve new ojectives/complex tasks. Seeks feedback.

Creates solutions by applying their expertise. Supports team members.

Mentors others and develops trainings.

4. Senior Engineer.

Sets higher level objectives without direction.

Creates simple solutions to complex problems. In depth expertise and knowledge in their area of expertise, and understands core concepts in related fields.

Able to teach complex topics and fundamentals.

This list continues a bit further and I skipped a level here or two...

Where are you?

@Questor basically none. My boss will ask "hey, it would be cool if we had something that did x. Is that possible?" and then I go ahead and automate it inside the app so in the future

You are an engineer, on your way to becoming a lead engineer. The only thing you lack is mentoring junior engineers (by giving them tasks instead of doing it all yourself). Which helps to spread the knowledge around about the thing you developed so that your company has a better bus factor.

Am I being underpaid.

Yes. How many hours of work has your app saved? How many errors (beacause humans are more error prone than machines) have you prevented? Probably a lot, if you can figure out how much time you have saved, you can use that to argue for why you should be paid more.