I may be wrong but I think jeans have a french seam down the inside of the legs. If I remember right from my home ec days, the seam is sewn right sides together, one side is then cut lengthwise of the seam, cutting about half the width off. Then the other side of the seam is folded over the cut side and is stitched down on the outside. Hope that makes sense.
OH, SHOOT!! That's a flat felled seam I'm thinking of.