Put them on to find the most comfortable fit? And mark the back with safety pin or stitch of contrasting thread.

The longer piece is the back. At the hem, put all four seams together precisely, smooth the legs carefully up along the seam lines to the crotch as if you are trying to make sharp creases up the center front and back of the legs. You’ll see the longer back side.

As sheesh said, I would expect the back to have more fabric around the crotch seam. You might be able to see a difference if you fold them flat from the side as if you were going to hang them on a trouser hanger. The backside might have a little more room or outward curve in the fabric. Love maddie's suggestion to add a little contrasting stitch to mark the back and make it easier for you next time.

The center seam from the waist to the crotch will be shorter in front than the one in the back. So I lay it flat lining up the center seams and waistband and the crotch seam will be on the side that's the front.

Stitch a scrap of ribbon or button inside the back of them to make it easier to tell the difference in the future.

I have a bateau neck top that I had to remove the tag from. It itched. I put a little x in the back with embroidery thread. I’ll probably do the same for these pants now.

it makes you seem mysterious too ... bespoke clothes, no tag, just "her mark"

To whom exactly, IDK!