I would like to check that my rotary encoder switch is working. A video on YouTube says it can be checked using an oscilloscope but doesn't say how to connect it. I guess from pinouts that I've seen I need to connect the two pins to Vcc and GND and the other three to an oscilloscope with common GND in the middle. Are there other components required? How do I determine which pins are positive and negative? Would it be damaged by reverse polarity? Thanks.

 

Try this...

The switches in these are not damaged by reverse polarity if it is these cheap ones.
View attachment 160517
Do not connect both Vcc (+5V) and GND (0V) to them.
Otherwise you will short the supply out as the switches operate.

 

Thanks very much. For the third of the three pins, the LED does not flash (confirmed by swapping leads around). It flashes for the first and the other one comes on when the encoder is depressed. I guess that means the third (top right in your circuit diagram) is faulty.

 

Thanks very much. For the third of the three pins, the LED does not flash (confirmed by swapping leads around). It flashes for the first and the other one comes on when the encoder is depressed. I guess that means the third (top right in your circuit diagram) is faulty.

It does sound like it.
The top 3 pins are a Quadrature waveform generator so there should be 2 square wave signals produced when it is turned.
Have a search for Quadrature Encoding to see what it is used for.