It’s really hard/can be impossible to tell if cabbed turquoise is stabilized just by looking at it, and even more difficult to to tell just by looking at pictures of it. Sometimes with raw or nugget turquoise you will see brownish clear stabilizing residue, but you won’t see that in a cab. I think the most reliable way to tell is to use spectroscopy, but of course most of us don’t have access to that. If it changes color over time it’s natural, but that doesn’t help us when we’re buying. If it has a very shiny glass-like surface, it might be a clue that it’s stabilized, but high grade natural stones can take a good polish too. I’ve heard people say that when you cut stabilized turquoise it has a telltale plasticky smell, but that also doesn’t help most of us who are not cutting our own turquoise. I think you might find this old thread helpful:
In it one of our members describes soaking cabs in water, if it darkens after a couple of hours with water penetration that means it’s not stabilized. That could be helpful if you’re buying stones; I haven’t tried it yet personally. Note he also mentions stones can falsely fail if they’re already impregnated with grease or oils.
So I know that it can be hard to “know your seller” as chicfarmer suggested, especially when you are just starting out, but really it is your best bet. Buy from reputable sources, and if possible directly from whoever mined and cut it because then their information will not be second hand. Another strategy would be to buy older jewelry. I don’t think stabilization was common until the 60’s, so any turquoise that was set before then should be natural. But then you have to learn to date jewelry, which is a whole other skill set. 
Or again, just buy from knowledgeable, reputable dealers.