I have several seconds, a few of which I thought were in better condition than the firsts.

There is a small mark (3/16") scratched in the enamel under one of the handles (which looks like it was made with an awl or icepick)which looks like a backwards 3.

If you weren't looking for it you would never notice it.

Thank you, windje--I checked the underside of the dish and its lid, but couldn't find the little backwards 3. The dish is a deep-dish baker, so it has no handles. I thought I might find the little mark somewhere on it anyway, but didn't. The reason I ask is because there is a small rectangular, ever-so-slightly raised area on the bottom of the dish, just beneath the Le Creuset impression. It looks like someone has scratched something off in this area (I got the dish on EBay). It has straight edges, and it's about an inch across and maybe a half inch from top to bottom. It looks like it might have had some writing in it? Not sure, but it definitely appears that someone took some steel wool or something like that to "sand" something off in this area. Thought it might be a "Seconds" marking--the seller guaranteed the item was a first. The dish has a slight wobble when set on a flat surface, and that's what got me wondering.

Any idea of how Le Creuset marks baking dishes?

No idea - but I'll bet if you call one of the outlet stores they would tell you - its no secret as they need to have some system to prevent seconds from being returned for credit as firsts.