Some airlines may operate the same type but with 2 or 3 different configurations for specific sets of routes. As a small example, Qatar A330s for instance have 2 basic configurations: F/C/Y and C/Y. They have routes where they only offer a two class cabin and therefore (with the odd exception due to an a/c being u/s for instance) these routes will not see their 3 class cabin configured aircraft - and v.v. I guess if you used their reservation engine you could see what cabin configuration they offer for each destination. Some enthusiast airline fleet publications also indicate the actual configuration of each a/c in the fleet.