Don, the way I do it is to jack the front wheels up off the floor just so they can be spun . Than I use a flat screw driver to scribe a line in the center of the tread as the wheel spins. I prop the screw driver up on a jack stand to hold it still as it spins. Make the line all the way around the tire on each front tire. You can use the inside of the wheel or the inside of the tire to measure from but it is more accurate making a true center line.
Next let the car back down on the ground and measure from center of line to center of line on the front side of the tire tracks and on the back side of the tire track.
You may need to jack the frame up a little to clear the frame and motor hanging down in the way to be able to pull the tape across in the front and in back of the tires,keeping the tires just touching the ground.
I set my tow in at 1/16 to 1/8 tow in.
Of course all this is done if you do not have access to a older tow in gage......
Or you may be able to make a gage as some one will hopefully chime in after me and show the tool that they made.
Hope this will help and I hope you under stand my instructions.
Ron