Like others, I've never been asked to show proof. I say self-employed and 50k. The solid credit score gets it done.
Funny thing - I can recall from college being bombarded with offers (20 years ago) . They would throw in a free T-shirt or some incentive to just get you to apply. Well, often, I liked the incentive, but didn't want a new card. For income, I put $0....certain that I would be rejected. Nope. Approved.
Housing bubble, etc. put an end to those days, I think. My wife applied for a card recently and put $0 and they did reject the app. Her credit score is around 800, too.
I think any minimal amount of income will get it approved.