There is no proof for any bottle of wine. You can open it and it has gone bad.
Good is subjective. Well flavored is subjective. Flavor is more about the grape. If you don't like a Cabernet Sauvignon then you are not going to like an expensive Cabernet Sauvignon.
There are numerous reviews and point systems.
A vineyard will have a reputation.
The wine buyer at the store has the reputation of the store to protect.
Restaurants will sample the wine before putting it on the menu.
Yes wine snobs know the difference and they LOVE to talk about wine.
If you are not a wine connoisseur you might not know the difference between a $40 and $400. But you would probably prefer the $40 over over the $4 of the same grape.
In a free market supply and demand establish the price and wine is a liquid (that is an economics term) commodity. Even a small run will have a set of knowledgeable buyers.
Yes they may have just slapped a price on it as a marketing ploy. Does not mean the market will bite. For sure you don't need to bite. There are many reviews available. If you want to get into higher priced wines then find reviewers / publications you trust.