A dual check valve is similar to a double check valve in that it is a mechanical backflow preventer consisting of two independently acting, spring-loaded check valve.

There is no difference between check valve and NRV, both are same

A check valve allows for flow in one direction and a gate valve is used for positive shut off and a globe valve is used for throttling

A check valve or one-way valve enables fluids to flow through in a single direction.

You can check the valve clearance with a valve clearance guide. Slide the guide between the valve. The guide will register the clearance.