The truth is, in many instances when buying leather goods online, you can’t actually know for sure if it’s real leather or not, just by looking at images and reading product descriptions. And it's a common practice by many online and physical retail shops to not give detailed product descriptions. This is done for the simple reason that leaving customers thinking what they will may just be what drives the sale.
In such a common situation it helps to contact someone from the website and ask questions. You can ask questions like, “Is this bag made of real leather?” or “Is this made of another material but labeled as leather?” or “Can you tell me about the kind of leather this bag is made of” or “Is this bag made completely of real leather” or "is this bag made of a leather that comes from an animal hide?". Usually the last question is where you will get a straight answer, and many times buyers find that the bag of their interest isn’t made of real leather or it isn’t entirely made of real leather; it’s just called leather in the product title without any other information about what part of the bag is actually made of real leather.
An honest retailer will give you all of the information you need in the product description, and product images will be large enough to support the details and their claims.