The basic pale blue jasperware has gone off the boil rather Rose, but the better stuff only has Wedgwood on the base (which means it was made before 1891) , or Wedgwood England (which makes it between 1891 and +/-1910).
Also what is called "dipped" jasperware, the stuff with the white inside rather than the same colour, is more sought after, together with different colours. I have a piece of lilac jasperware for sale at the moment, which is very pretty and desirable.