The trivial way is a phone call to a friend in a different time zone. It's dark there; it's not dark here. It's just past dawn here; the sun is at the highest a few time zones east. More phone calls to more places and it's pretty quickly apparent that the angle to the sun is exactly what you'd expect from a sphere and not from a disk.
Not that it matters. Flerfers have had decades coming up with increasingly elaborate and absurd cosmologies. They don't understand them, but that's what science looks like to them as well, so they figure it's your word against theirs. And since their word is the Bible, they win.
If they were genuinely curious, there are a million ways to show the roundness of the earth. But there are zero ways to demonstrate it to a person who is fundamentally incurious and doesn't want to know.
