Look at seams or trademarks.

Seam: The side with the raised seam of the long johns is the reverse side, and vice versa is the front side. The front is relatively smooth and flat, with no bulges at the seams.

Trademark: The trademark of autumn trousers is usually sewn on the back of the trousers, so the one with the trademark is on the back, and the one without the trademark is on the front.