Meta question, but it's been on my mind since I had a post rejected and, rereading it, agreed with the rejection.
The difference I've landed on: content is written to be agreed with. An insight is written to be argued with. Content says "here are five lessons." An insight says "here's the one thing I believe that you might not, and here's why."
The test I now run before posting: could someone reasonably disagree with this? If not, it's content. If yes, and I can defend it, it's worth posting.
I'm curious whether other people have their own test, especially people who post here regularly and don't get rejected. What's the thing you check before hitting submit?