They're proliferating at a ridiculous pace. People on here keep linking to them as suggestions. I'm generously assuming they do it in good faith but, if so, that shows they've been taken in!
First thing. Check the returns policy. If it has a lot of words saying you will have to return your purchase to an overseas depot after messaging them to get the address: avoid!
I like to look up their head office on Google Maps. Scammers use random addresses, usually nothing like a business premises or fashion showroom. I've seen one that was a demolition site, and one was a school. Most are just normal flats and houses.
Adorable origin or closing-down stories. Have you ever seen a legit fashion company showing you their children, parents and dogs? Nope, that's for bloggers and 'influencers'. Fashion stores sell fashion, not their personal lives. Same goes for sad tales of closing due to personal circumstances, which are explained in detail - and identical to the sad circumstances of another closing-down sale or ten.
Multipurpose models. They buy packages of model photos, which can be virtually dressed in any garment. Even garments that don't exist - they'll knock up something vaguely similar if you order one. See the one below? They've used photo A three times, and didn't even notice they'd cut off her arm in two of the images!
Same dress, different colours. Sure, makes sense. What doesn't make sense is the photos being identical. If they're real dresses on a real model, the pictures will be different. If it looks like they just photoshopped a different pattern on the same picture, that's because ... they did.
TheTrustpilot browser extension floats quietly in the corner of your screen. You can click on it to see recent reviews, and it links directly to the reviews page for the site you're looking at.
I'm sure other MNers have more tips, including how to get our money back if we've fallen for one of these. They steal your data, too - I know others have more info about this.
Stay safe!