Tools, Sharpening and Power Machinery - How does one check if one's level is true?

I don't have any tubing handy to set up a water level, but I need to check my bubble level.

place it on a level surface and then flip it 180 and see if it reads the same

The simplest quick check that I utilize.

lay the level on a surface. Take note of where the bubble is. Spin it (not rotate) so what was top is now bottom. Compare the bubble position. They should be in the exact same place.

Now rotate the level end for end. Check the bubble again. It should be in the same relative position in the tube. Now spin it over again.

The bubble should should always be in the same place as you look at it. You can check like this on an unlevel or level surface becasue you are comparing bubble placement, ot levelness of a surface.

You can do the same for the plumb bubble only use a vertical surface.

If the bubble ends up in a diferent spot, you have a problem. Can't tell which surface or vial is wrong but just that there is a bad level.

As an electrician, I utilize a pocket (torpedo) level. Each time I buy one, the clerks look at me like I'm nuts as I check each one for the best consistancy,

Well that's certainly a lot quicker [and neater] than setting up a water level. Thank you.