Price: If the price was expensive in the first place; most likely, it will remain expensive especially if it is in mint condition. Remember the trends for precious and new items. The latest releases are always expensive and the older the piece, the more precious it is. This is the same for jewelry, shoes, bags, luggage and dresses. Especially for couture, the older, and the lower the runs of the piece, the more expensive it would be.
Do some era-based research: If you are familiar with the specific silhouette of that decade, then it will be easier for you to know what makes a piece special or not. The 1920s have a more relaxed, look, with the popularity of the flapper dresses. Ornamentation are also popular. The 1950s is the era of the New Look, a reaction against the modest during the times of war. Each decade has a specific set of designer popularity. Items from the 80s might include vintage Claude Montana or something similar.
Study the future vintage: Alexander McQueen is a popular collectable piece. Even if most of his pieces are not considered as vintage being less than 25 years or so, they are notable collectables. Read books. The recently concluded Met Museum McQueen exhibit resulted to a lovely book with notable pieces. You might want to start with those. Read news about notable collections. The last collection of Olivier Theyskens for Nina Ricci is definitely vintage worthy so you might want to snag one of the shoes or dresses but they can be harder to get nowadays.
Study the labels extremely well: Something that might look like a Madame Gres may be something that was actually a pattern piece bought by department stores from designers. It is possible that the designers licensed companies to use their designs. Therefore, a specific design may look like a Gres but it is not an atelier creation so do not be mistaken. Some shop owners may lure you to saying it is a Gres but they just placed a label at the back. If you are uncertain, have an expert with you to look at the piece.