Kevin has used a regular level a couple of times, holding it with two fingers on a short side to dangle it. You look inside the sight glass and make sure the bubble is between the two hash marks in the lower tube. Now you know that the long side of the level is vertical.
You can hold a level out, get the bubble in place, and then just eyeball whatever you are working on to see if it is level in relation to the side of the level.
The other way to cheat – and it’s been around for a long, long time – is using a heavy teardrop shaped tool. It’s not a bomb or a bullet, it’s a plumb bob. A plumb bob is a weight with a tapered point at its bottom and some way – a cable, string, wire – to hang it at the top.
When it hangs, a plumb bob shows you what is vertical. It can’t hang at an angle – it has to hang straight.
What if you don’t have a plumb bob? Improvise! Use a straightened coat hangar with a rock, a spike, or anything heavy enough to make the cable, string, wire, etc. hang straight. Now you have something vertical to look at.
A plumb bob is really old technology. Sometimes that’s the best: no batteries, no owner’s manual. You can make one or, if you just happen to find an old one at a garage sale ….
Or if you’d like a better look at that old brass plumb bob ….