Google has recently added yet another occasionally handy tool to its search box, if you're searching from a phone. Just type in "bubble level" and you can get a box that uses your phone's accelerometer to determine if something's level or not.
While it's obviously possible that a digital representation of a bubble level could be incorrectly calibrated and thus slightly off, this tool still does the job in a pinch. In my tests it was accurate to at least within one degree. If you need reliability in your everyday work, you're probably already carrying a real bubble level with you, but for those one-off times when you can't find your level, your phone can do in a pinch.