An encoder is sometimes referred to as a position encoder, a shaft encoder, or a motor encoder. Encoders come in all shapes, sizes, voltages, and pin configurations. For those trying to find out what is the easiest way to test an encoder, I have some suggestions on the matter, but not one simple test to cover all manufacturers and encoder styles.
(http://www.obsoleteindustrial.com/photos/easiest-encoder-test.jpg)
First of all, a large percentage of encoder problems (I'm not sure of the exact figure) aren't the encoder at all. It is a cable connection between the encoder and whatever device it is providing to ( a drive, controller, PLC, etc). Broken wires and bad connections in the connector can be a nightmare, or they can be obvious. So check your cabling first.
Secondly, a mechanical connection between the encoder and the motor is a weak point that is known to fail. Either a loose set screw, broken or misaligned coupling is another source for loss of position reporting. So check the mechanical connection next.
Next you may be able to try interchanging the encoder you suspect as failed, with a known working encoder. Either off the shelf, from another point on the machine, or from another machine all together. This is a simple process of elimination and is the closest thing to the easiest way to test an encoder.
If you're the type of person who likes to have test equipment on hand. There are various encoder testers commercially available also.
Check eBay for some ideas. (http://rover.ebay.com/rover/1/711-53200-19255-0/1?icep_ff3=9&pub=5574630700&toolid=10001&campid=5336103693&customid=&icep_uq=encoder+tester&icep_sellerId=&icep_ex_kw=&icep_sortBy=12&icep_catId=12576&icep_minPrice=&icep_maxPrice=&ipn=psmain&icep_vectorid=229466&kwid=902099&mtid=824&kw=lg)
If anyone can add to these suggestions, that would be great...
From a maintenance technician's standpoint, the best defense is a spare encoder. It is easier to interchange encoders than it is to perform complex electronic evaluations. In other words, swag encoders now and ask questions later.
Good info, I thought it was worth bumping.
(https://www.obsoleteindustrial.com/i24/encoder-fault-test.png)