How do I detect live electrical wires with a degree of confidence?
I would treat an inconsistent reading as a positive (live wires).  The reality is, it's hard to detect with 100% confidence an actual live wire behind a wall with a stud finder or non-contact tester as these can be triggered by low voltage lines, or line-voltage lines that are off but in close proximity to other live circuits.  The proximity of the wire to the walls surface is also a factor here.

Wires running vertically should be stapled to the stud so you are less likely to hit them when cutting into walls, so it's really the horizontally run cables you need to worry about.

An inspection camera is a handy device for these scenarios since you can look around the stud cavity to confirm or rule out the presence of electrical wires.

I'm assuming this is because you want to cut into the wall.  If you think there might be wires behind where you are cutting, instead of using a drywall saw, try just scoring the drywall and give it a good whack with a hammer to break out the piece of drywall - you're far less likely to damage wires or other infrastructure this way.