One thing to try is, if you have a perfectly level worktop or bench then using a carpenters square lined up with the recoil pad screws can sometimes be an option.

Another is by using a piece of flat bar, say 1/8th or 1/4 inch by a foot or so, and balancing this on your known flat on the rifle. This allows you to use a slightly larger, and closer to accurate, level. You just extend the flat bar as a counter balance against the level.

Finally, when you know your rifle is level it's best to set the scope using a plumb line.

Not very scientific I know but it gets you close enough for the real world.