How can i check to see if my gauge is bad or my sending unit. Whats happening is at full tank it reads 3/4 and empty it reads 1/4. thanks dom

Take a look at this (http://www.chevelles.com/techref/ftecref18.html) article by Wes Vann.

sender

Follow the instuctions in Wes's article to see if it's the sender or the gage. If the gage only goes to 3/4 tank when the sender wire is open, there's a chance that the problem is the resistor on the back of the gage.