Not sure of a test for the PCM/ECU, but the guage cluster can be checked (sort of) by removing the single wire connector. With the key "on", the guage should be at the cold mark. Ground the wire, and the guage will read full hot. This lets you know that the cluster guage is good, along with its wiring.
If the sensors are old, it would not hurt to replace them both. They are only 10 to 15 bucks each. As it has been mentioned in other threads, do not use any type of tape sealer on the bottom of the sensor threads; they require a good ground contact. You can place a small wrap of tape sealer at the top of the sensor threads.