1) Does it emit radiation? If so, it's probably something you don't want next to your skin a lot. Remember that the most common danger from radiation isn't a single big does, but the buildup over time, in which a lot of tiny doses definitely add up. This could be greatly reduced by shielding. Companies probably won't bother unless consumers force them to. But you can probably make your own. Radiation from wearables isn't very strong so you don't need six feet of concrete to stop it. Consider also that some wearables emit a lot more than others. Don't choose a high-rad device if a low-rad one will do what you need.
2) Does it contain toxic materials? This is common, as things must be proven unsafe rather than proven safe. There are things which, again, you probably don't want to press on your skin all the time. This is easily solved by using a protective cover of something you're not allergic to and don't make go haywire. Don't trust corporations to put your safety first; they are legally obligated (in America) to put their shareholders' profits first. Don't trust government agencies either; they're interested in avoiding panic and making their donors happy. You are not their priority either. Look for reports by people who don't have a dog in the fight. These are rare, but tend to be more reliable if you can find them. Also if something is getting banned somewhere, it's probably not great for your body.
3) Does it interfere with your somatic motions, make your body part hurt, or cause some other physical problems? This is likely an ergonomic issue which may be affected by the size, shape, weight, etc. of the device. Later generations will probably improve. If you have this problem, try moving the device, carrying it in a pocket, etc. If all else fails, wait a year or two and test a newer model.
4) Does it mess with your social, sleep, or other life patterns? This tends to be a behavior issue, which is a combination of physical and nonphysical factors. Staring at a glowing screen at night can make you unsleepy (but not untired, alas). So can thinking about complex things such as what someone's latest message means for your schedule tomorrow. If you have this type of problem, try modifying the times when you use your device. Staring at the gizmo on your wrist could cause your to walk into traffic and get hit by a car, or piss off your friend and get dumped. If you are having this type of problem, try changing the way in which you interact with your device and with people around you.
5) In any case, pay close attention to new technology before and after adopting it. What are the financial, physical, social, and time costs? What are you gaining by using this tool? What are you losing? I highly recommend the Amish rule: When deciding whether to adopt a new piece of technology, ask whether it does more good than harm. If it does more good, adopt it. If it does more harm, do not adopt it. (You don't have the draw the line in the same place they do. I don't, but I use the same rule. Heck, even they argue over it.) Really really think about this. Don't grab something new just because it is new and everyone says it's cool. Think about what it will do for you and what you'll have to give up to get that.