A good test if, if you're not losing coolant or having run issues, you're probably not over heating.
My fan turns on after a bit of riding (~5-10 min), and that's one of the indications that it's at full run temp. Once my bikes fully warmed up , and I continue riding, the fan mostly stays on as far as I can tell.
If you were actually overheating then you'd probably start boiling coolant which would splash all over the place when riding.
Summary, I doubt it's overheating, but if you want you can probably get a bolt-on temp sensor or even take external reading with a laser sensor (which won't be as accurate).