There is no check valve that i know of, but a vacuum breaker usually located in the bathroom under the sink. (Model specific.) They are made of plastic and usually fail and leave water on the floor. most of these end up replaced by ether removing the old one and putting a screw on hose type in the convenience center or just replacing it with a good brass one under the sink. You can find it as the pex line is usually colored white. but as you stated it could of been removed.
2017 Ram 3500, Big Horn, 6.7, Crew, Long, DRW, Aisin, 4:10. Full timer in a Pinnacle