Gas Gauge

Well I got up this morning to go see my Chiropractor
And as I was driving down the drive way
I noticed that my gas was gone:angry7:
Jeeees I know I had over a half a tank.
so I drove back up the drive way and looked to see if and tracks of anyone steeling my gas last night. No foot prints to be found I shook the car and herd gas so I drove to my appointment.
On my way home I filled up my tank and it held 5 1/4 gallons of gas.
I just changed the shocks on the car and I am wondering if I did something to it then, How is the best way to test the sending unit?
Remember this car is 44 years old, The float could be setting on the bottom.
Or my gauge in the dash is not reading the float sender.
as a remider I wrote down the milage at 56,996 at fill up.:-D

What would be the first test and how to ? Thanks for your time.