I've had juices that claimed they were high VG and just looking at the viscosity, it was obvious they weren't. Pure VG is thick and almost gooey. It should coat the bottle when you tip it. When it gets cold, it runs out and doesn't drip out in drops.
That doesn't mean PG is used in the juice. Water and alcohol too thin out glycerin.
My juice supplier is far from one of the many fly-by-night shops I keep reading about here on ECF: it's a offshoot of the chemistry department of a well-know French university, and that pretty much guarantees they know what they're doing when they prepare juice, and what you read on the bottle's label is what you get. In fact, that's one the main reason why I get my juices from them and not from random dudes who happen to have a website and a Paypal account.
The other day, I was looking at a bottle of 100% VG pistachio-flavored juice from them, and found it very runny for something that's supposed to be pure glycerin. I sent them an email asking for explanations, as I'm intolerant to PG and don't want more than strictly necessary in my juices (there is some in the flavorings). They answered they thin out all their VG juices with water to make them vapeable as-is, and that no PG was used in my juices, as per my instructions. I believe them.
In any case, I thin out my own homemade VG juices too. The straight stuff just doesn't wick very well in most clearos at room temperature.