One of the reasons I started my writing company is precisely because of situations like this. Hiring a competent writer is difficult as it is, but when you combine that with scams for stealing content or people taking advantage of fledgling writers, it just makes a bad situation worse.
As someone who regularly hires writers (and for way less than they are actually worth), the best advice I could give would be the following:
1) Writing samples that are already published are more than enough for anyone looking to hire writers. Period. All samples tell a person looking to hire you is what sort of writer you are. What is your voice, what is your style. If they like what you produce as a writer, they'll move on to looking at some various other factors that they care about.
2) The worst sort of clients are also the worst sorts of employees - hagglers. If everyone is being honest, the rate is the rate. Either you are willing to take the rate, or you are unwilling to take the rate. If someone is trying to get the cheapest price they can, that's fine mostly, but once you have drawn a line in the sand for how little you are willing to work for (which probably reflects how badly you need the work), people should be willing to respect that and either walk or way or cough it up. No harm, no foul.
I encounter this all the time, seeing writers that I deserve more money than I can afford to pay them. I try to balance this out in other ways (such as making my numbers transparent, making the environment flexible, and arranging it so that the company operates as if it were owned by the collective of writers working for it rather than just me). Sometimes this is enough to make working for me worth it, and sometimes it isn't. As long as nobody's taking it personally, then we can expect everyone to act like professionals.
But working on spec is BS and should not be asked of anyone.
3) Don't be afraid to be honest. If you are concerned a company might actually be a scam, no business owner is going to be offended. In fact, they are probably worried that you are going to scam them in all honesty. Addressing the elephant in the room is a good way to clear the air, and the middle ground should generally be half up front and half on delivery for a project. At some point someone needs to take a risk, and asking everyone to share the risk equally is great.
However, there are ways you can mitigate even that.
If you find out where your content will be published - which should never be a problem - then you have a place to file a DMCA claim against them. That's bad news, and no company will want to have to deal with that.
Finding out their business information is important as well - no company should have anything to hide about how they operate. It might seem weird to ask at first, but its as simple as saying, "I'm sorry I have to ask that, but you'd be surprised at the number of folks that are dishonest. If it wasn't for a few bad apples, this would be a lot easier than everyone because there wouldn't be so much due diligence required. I'm sure you feel the same way about hiring writers, lol."
That works, because its true.