When the sun sets, the sun sinks behind the horizon and theoretically is lower than the clouds that I'm standing under (and to the east of me). The underside of the clouds should be illuminated while the tops should be dark. However, I've observed every sunset that the TOP of the clouds are illuminated all the way to the darkness of night (the bottoms are never illuminated), indicating that the sun is always at a higher elevation than the clouds, meaning that it's not rounding off behind a horizon, it's merely getting farther away.

If you use a super zoom camera at the edge of a large lake, you can clearly see wayyyy further than you should be able to on a globe earth. There's several videos showing the shoreline at 14+ miles away, which shouldn't be visible at all. Pretty solid proof of no curvature. This is also in several countries around the world, so it's not a localized anomaly.