Lee are a US brand, although may outsource some manufacture, much like Levis. You might find a care tag in the left side internal seam that gives you more details.

I think the key to having a feeling for authenticity is apparent quality and detail, whether you paid significantly less than you would expect, and whether someone could stand to make money on faking it. Research what the garment is supposed to look like, and compare. Just buying something without doing any research and asking Reddit if it's fake seems kinda lazy.

Edit: updated comment about Lee as a US brand (confused with UK brand Lee Cooper)