A million green flags. The greenest of green flags.
An early one was finding out that in his mid-30s he was still close to 2 guys from h.s. (one from elementary school) despite them all having college degrees, established careers, and living in various states. They graduated h.s. in the 80s and immediately left their hometown, but kept in touch without the modern conveniences of cell phones and social media.
It showed me he was loyal. If he was willing to invest in those friendships for decades I knew he'd be more than willing to invest in marriage. (Next month we'll mark 24 years together. We've been married 22+ years)