The phone app and conventional levels are all OK, but for about $4 on a Stern machine you can add a bubble level that will never move:
https://www.pinballlife.com/stern-bubble-level-assembly.html
For Bally and Williams it is like $15
https://www.pinballlife.com/bubble-level-assembly-for-williamsbally.html
I am a big fan of these...
If you bump the machine and move it a little, is it still level?
Just check the bubble...