What do you use in the field or at the range as a reference to "level" your gun/scope/crosshairs before taking a shot?
The whole point of properly leveling your scope/rifle is for a reference point. If you have a rail mounted level, your scope needs to be level to the rail mounted level. If you have a scope mounted level, then the scope needs to be level to it. If you are not using a weapon mounted level(be it rail, scope or other) for reference then your just P!$$!NG in the wind.
I use a scope level in the field.
You can use a tall target test to insure that the scope level the level with the scopes tracking. You can do this by snapping a chalk line on a 8' 2x4 and standing it with the line plumb at 100 yards. Draw a target closer to the bottom of the board. Shoot a group with your scope zeroed at 100 yards on the bottom target. Now dial up until you top out your scope (or a reasonable amount like 50 MOA or something) and shoot another group. You should be right on the plumb line. If not your level is off.
I built a scope tracking test rig so I adjust the level with the scope mounted to it. I have the same target with a 96" plumb line on it and I line the cross hair up with the line and run the scope through the elevation travel. If I dial up 70 minutes and the point of aim has moved off the plumb line then I make an adjustment. The scope level is level with the scope when I can dial up through the range of the elevation that I have available to me and it stays on the plumb line. (This is one reason I really like Flatline Ops levels. They have set screws that are used to finely adjust the level.)
Using cards under the scope to level the scope to the rail is pissing in the wind.