how do i find out that magic mouse is genuine or not?

If you aren't in the possession of the magic mouse, ask for the serial number, confirm with apple it is a genuine serial. (still can be counterfeit, though)

If you have the mouse (and a mac to connect it with), connect them.

Open "About this mac" under the apple-menu, to-left of the screen, click on the bottom-left button saying "System-..." (I'm on a german mac and don't know the correct expression).

A window opens with a list of every possible stuff connected/built in. Find "Bluetooth" and there find the magic mouse.

I think I just cannot verify this.

You have to.

Connect with apple (in a store for example) and ask them.

Or connect the mouse and look it up in the system-profiler (the method described above)

I guess it boils down to where and from whom did you buy (or are thinking about buying) this product? If you bought it on eBay, Facebook Marketplace, or some other social media then all bets are off. If it is being offered at a price too good to be true that’s another red flag.

Unless you buy it directly from Apple or an Apple authorized reseller you can never be sure. Crooks are always ready to make a fast buck on users looking for a ‘deal’.