Lay a pair of well-fitting pants flat on a hard surface
Smooth out any wrinkles
Measure from the crotch seam where the inseams meet
Run the tape measure straight down the inside seam of one leg
Stop at the bottom hem of the pant leg
Record the length in inches or centimeters
Repeat on the other leg if needed to confirm accuracy
Use the same method on pants you already own for comparison
For best results, measure a pair of pants that fits the way you want the new pair to fit