It’s taken me a couple of years to find and recognise White wagtails for myself. I would hear other birders talk about them and look at the photos they posted, but not really see any difference between Pied and White wagtails. Finally, I found some and it was immediately obvious they had a different look.
To be clear, these are not two different species of bird: Pied Wagtails (Motacilla alba yarrellii) and White Wagtails (Motacilla alba alba) are different subspecies of the same species, which rather confusingly is known as the White Wagtail (Motacilla alba).
In Britain, we mostly see Pied wagtails, with the Whites passing through during the spring and autumn migration periods, and the White wagtails tend to have a much lighter grey back and a very clean, white belly and flanks.
Thanks to an excellent blog post I found, I discovered the reason why it was easier for me to tell these particular Whites from the Pied wagtails they were with. The blog has much more detail but, essentially, the Whites complete their moult sooner (usually by the end of August) because of their need to migrate in peak condition from northern latitudes (most of our western Britain White wagtail migrants pass through from Iceland to the southern Mediterranean and Africa, and vice versa), whereas the Pieds, being mostly resident in Britain, don’t complete their moult until mid to late September.
So, the plumage of the Whites I saw looked clean and crisp and fresh, whereas the Pieds were still looking rather scruffy, as you can see below.