How do I tell yhe difference between plain and self raising flour

If you mean while it is still in the package, it will be called all pupose flour for the plain, or self rising for that type. Once it is out of the package, the site below suggest that the self rising flour will have a salty taste to it.

Sara Moulton

Obviously a fellow brit there ;)

Plain has no raising agent and self raising does hense why you use self raising for cakes etc.