Most temp senders are ntc sensors, meaning as the temp goes up the resistance goes down. You can check the guage by grounding the sender wire and the guage should peg out at max hot. That will tell you the guage is working. Testing the sensor is harder you need hook up a dvom (ohm meter ) and check the resistance of the sensor to the case with the engine
cold, then warm up the engine and check again. You can often get the specs for the sensor by searching the product number that is cast into the sensor, but you are looking for a change in the resistance . As for where the sender is you would need to look for it, you can look here andtry ot find a picture.