First things first. Check to make sure the gauge itself actually works and there are no problems with the wiring. You will find a 1 wire hookup sensor to the right of the distributor, this is the temp sending unit. Looks something like this.
Take the wire off of it, and ground the connector out somewhere with nice clean metal.
This, if all is well, will cause the gauge to go up to full hot, as far as it can go. Be sure the key is on.
If this does not happen, you have a bad wire, or a dead gauge.
If it DOES happen, you need to replace that sending unit.