They are called reticulocytes, because of a reticular (mesh-like) network of RNA that becomes visible under a microscrope with certain stains. They appear slightly bluer than other red cells when looked at with the normal Romanowsky stain. Reticulocytes are also slightly larger, which can be picked up as a high MCV (mean cell volume) with a full blood count done by machine.
Reticulocytes increase in number and percentage when there is a lot of new production of blood cells, (such as in a haemolytic anaemia).