There are a few ways.
Most have a sensor that will sound if the o2 drops blow acceptable levels.
The machine manual can often be found online or from the manufacture.
There is also an after market sensor you can buy.
Or you can play mad scientist and run a few experiments.
All you really need is a test tube, some OOOO steel wool, and some acetone.
http://chemlabs.uoregon.edu/Classes/...centOxygen.pdf