There are retailers and there are marketplaces. Marketplaces are always going to have a chance to be fake some other fuckery. You can also check any brands website.

Retailers will almost always sell the authentic product. Walmart will carry the brand they advertise because if Walmart sells a counterfeit of that brand then they're not distributing the product they're contractually obligated to. Any store with a brick and mortar counterpart is generally going to be a retailer and thus you'll have that assurance.

Marketplaces invite other sellers to come in and sell via their store, and some massive brands might have some guarantee you can get scammed on some other products. Marketplaces include Amazon, Newegg, Ebay, Etsy and most other online only stores that carry multiple brands. Marketplaces are nice because you can quickly search product types and compare options, and sometimes they have really easy and hassle free return policies.

The best assurance you can get however is find and buy directly from the brands website. This is good to do for many reasons. It rewards good brands because they don't have to pay a cut to the marketplace, reseller, and/or retailer, you might get additional warrantee protections, and you KNOW you're getting the real thing. Downsides of this is you're might not going to get as fast and/or completely free shipping and sometimes return policies on sites like Amazon are so hassle free that they can be preferable if you're iffy about a product you're buying.

Unless I can find a much better price from a retailer or marketplace I will generally buy directly from the brands website if that's an option, barring that if you must buy from a marketplace there are ways to see if it's being offered through an authentic dealer or a potentially shady reseller.