How to check my phone is genuine

There are a couple ways I can think of. First, you can open Settings > General > About then see if the information provided is what you would expect to see - the right Model #, etc.

Next, open Settings > General > Software Update. Try to update the iOS if you are not already on the latest available version.

Finally, you can take it to an Apple Store and ask someone there.

I’m curious … why are you asking? Is there a problem with your iPnone?