How can I check my iPhone genuine

I’d be much more concerned about Activation Lock, and about Carrier Lock, and buying a lost or stolen or IMEI blacklisted device. None of which can be easily nor reliably ascertained remotely, unfortunately.

Here is what can happen: The All Too Common SAD Reality of Buying … - Apple Community

Well-known online sales websites are unfortunately all too common sources of lost or stolen devices, and the buyer there usually ends up with an expensive brick to recycle, and the seller is long gone.

Don’t buy a device from a third-party seller you don’t trust, or—if you’re willing to speculate—do expect the purchase to be a brick.

Please do not buy a used iPhone. You don't know what you are getting or what condition the battery might be in.

If you ignore this advice, at least make sure that the phone powers on to the "hello" screen. Anything else.....do not buy the phone.

A genuine iPhone will be able to connect to the App Store. A fake made to look like an iPhone, but is really an Android phone, won't be able to do that. It will either connect to the Google Play Store, or will show a message a connection to the App Store couldn't be made, despite the fact everything else on the Internet works.

Other than that, the advice from MrHoffman and Bob Timmons are both spot on. Be absolutely sure, at minimum, the phone has been reset as new ("Hello" screen comes up).