A straight edge across each wheel horizontally, and measure across at front and rear from one wheel to the other works to check toe-in. Have also done it by spinning each wheel and scribing a line in the center of the tire and then measuring. I also have a spring loaded toe-in gauge but it's not as accurate if the tire or rim has some runout. I've only used the string method to check alignment of the drive axle
to the steer axle
on trucks. Later we got a straight edge with a laser pointer for that and it was more accurate.