have a 2014 CFmoto 800 quad recently bought from dealer with LOW hours was/is garage kept.

The fuel gauge always shows full, any ideas what to check first? I had similar on a car and replaced sending unit in tank. And that fixed issue,

My guess would be just that, the sending unit in the tank might be snagged on something so that the float arm is always raised. If there were problems with the contacts on the sending unit then it would read always empty.
You could take the cap off and see. The float arm will operate off the fuel pump shuttle.