There's an entire list of ways to identify fakes. Some are easier to spot if you're a novice and others take experience with the brand to be able to identify. Here's a short list of some of the ways people check jeans:
1. Pocket stitching (are the pockets stitched correctly, shape, size, location, etc)
2. Wash quality (does the wash look like a real wash or is it plain, streaky, generic, etc)
3. Stitching (is the stitching inside of the jeans tight and even? This varies with discount
4. Tags (is the font correct? Does the wash code actually exist and match the wash?
are there misspellings?)
5. Denim quality (is the denim cheap looking/feeling)
6. Existence (does the style even exist? Some fakes aren't even imitating actual jeans)
Those are just some of the ways that people use but a lot of it comes with just being used to seeing a lot of real pairs.