Before calling the AG on this, how do you make sure that fake is fake in the first place?

The major hurdle is that you do not even know and cannot know. Maybe the product is actually legit and the visual difference comes from temperature or whatever, and the rash appeared for a different reason. One avenue may be to contact the manufacturer with the lot number.

If you try a new product and baby develops rashes or allergies, you know not to buy this product again. With Amazon you can't even conclude anything about the official product because you might have used a knockoff.

You can just conclude to stay away from Amazon for baby care products.