Goldy,
I used to do this by sighting across the center of the tire sidewalls toward the rear tires, but found my eyes to be less useful these days at depth of field. Then I bought the $9.95 alignment gauge from Harbor Freight. It is just a measuring stick with a support at one end and a spring loaded angle gauge at the other. Awkward, cumbersome and not too repeatable.

A friend lent me his device made from two extruded aluminum wall shelving supports - very inexpensive - one sliding in the other, held by a couple #6 machine screws. With belly pan off, he simply measures from inside wall front and back until, like Mr. Foster says, he achieves zero toe in. I have one car that needs a reset, judging by the look of the outer tread depth markers, so I'll soon get a chance to use this.

If my description fails, send me an email and I'll return a snapshot of the lightweight and inexpensive measuring stick.
--
Art Benstein near Baltimore