Doh! I didn't have any toolbars/buttons showing at all. That's the perfect answer.
The Synchronize browsing toolbar button is depressed when it is enabled.
I've not been able to find any evidence that this already exists, but my apologies if it does and I've missed it.
I find myself having to check if synchronised browsing is working regularly by clicking on a folder I don't need to visit just to see if the other pane switches folder too. Would it be possible to have something which indicates whether synchronised browsing is currently active?