Tim, if the lake outline is on the base maps, you will know it if you are on it. If you are not on that body of water, with an active GPS view, you can move the curser from your present location to the body of water location and see if it is there. You can move the curser to that location in the Simulator mode also to check.
Did you use Navionics with your Lowrance to get that body of water or did you use Lowrance Hotspots?