Hello,
How do I find out each leg of a Field Effect Transistor as Gate, Source and Drain. using a digital multimeter.

 

Try here...

http://www.circuitstoday.com/how-to-test-fets-jfet-and-mosfet

 

I simply look at the pinout on the datasheet.
Many, if not all Jfets have the source and drain interchangeable.