It's challenging--often a matter of trial and error. I saw your post on another thread about reading through pages of bickering, but given the anonymous nature of the game, that's often what it comes down to.

In general, I would suggest screening companies as if you were a customer, looking for broken English, etc. As you've probably figured out for yourself, be wary of any direct recommendations folks give here, as it can be hard or impossible to know who's posting and why. Instead of looking for reviews of specific companies, I might scan the forums for strategies you can use to identify them.

At that point, it might come down to taking your best guess, applying, and starting off by taking a few smaller, low-paying orders. If payment comes through as scheduled, that's a start--you can build up from there. Pay attention to the quality of English used in the interface and in your interactions with support, which can clue you in if the company is based internationally.

I can't speak for anyone else--and this is emphatically not a testimonial for any company, period--but in my personal experience, even fairly slimy companies will typically pay writers as agreed if your performance is up to a certain level. My (totally speculative) guess is that the business model just isn't particularly viable if you can't generate any return customers at all--and if you scam too many people, it gets harder and harder to advertise. That said, in all the bickering you mentioned on the other thread, there are plenty of horror stories, so who knows?

It can be a weird world to gain access to, whether you're a writer or a client. In either case, though, once you've started to make your way in, it should get easier and easier to navigate. But until you have some trusted contact, that initial process of trial-and-error can definitely be a stressful and unpleasant one.