How would you know?
Go back a few hundred million years and look at the fauna flopping around and you'll see all sorts of thins, including an unremarkable semi-amphibious fish with legs -
Tiktaalik roseae. It'd be a bit unusual but you can't recognise it as a transitional between bony fish and Tetrapods until you know that tetrapods are going to become a big thing and what features end up defining the group.
Pop back to the time of
Archaeopteryx and it'll appear as one of a number of flying dinosaurs, sharing some features with its close relatives and having a few novel features. It isn't until you've seen a modern bird that you can identify those features as a patchwork of features of theropods and birds.
It might be that, in the future, otters become fully aquatic and displace the cetaceans at which point a future scientist would look back and point out the remarkable patchwork of features that otters have, and their clearly transitional nature. But until we know what life will be like in the future we cannot identify transitionals.