I am pretty new to this and have been asking similar questions. Here is my take so far: reputation. You can analyze a vendor's website and get an idea if they know what they are doing, but anyone can build a website and copy/paste someone else's work. You can look at the documents they provide regarding the purity and quality of their ingredients, but not much I have seen is really worth the virtual paper it is printed on, if you are looking for actual proof.
You could theoretically buy products from a zillion vendor's and try them and send each one to a laboratory for full analysis, but not many of us have the time, resources, and motivation to do that. If you can actually visit the place where the juice is made and observe the operations, that would be great, but that is not feasible for me at least. You can call around to vendors you are considering and ask them about their methods and products, but a lot of people can talk a good game when it comes to making a buck.
So what is left? Reputation, and finding something you like the taste of and that doesn't do weird things to you, like cause irritation or make you feel stoned or something. So really, reputation is the first and best tool available. If you have friends that vape, talk to them. And of course this forum is the best place to check out a vendor's reputation; there are literally thousands of opinions you can check out. The only issue there is that people's opinions differ, and with so many opinions, it can be hard to find clear direction.
But there is some truth to the safety in numbers philosophy. If a huge number of people like something and continue to use it without ill effects, that is a better bet than trying something no one else has heard of. But in the end, you will still be taking your chances, so use all of the tools at your disposal.
