I bought Honda CB 400 SF, version S, used motorcycle. When I ride for about 15 minutes, the radiator fan starts running. While I am driving my legs felt warm air from engine. I am wondering if the engine is overheating.
How can I tell if this is normal?
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).
I had a CB400SF 2011 Model and experienced a similar occurrence. Although it is possible that everything is working as intended, I would recommend checking your coolant system. Ensure there are no blockages, and there is enough coolant in the system, and that the coolant is still effective in moving heat. Check that all the valves are operating. It may be that the engine is heating up, switching on the fan, but the valves that pass the coolant through the extended loop are remaining closed / are blocked.
