The causes of red eye are legion, but can usually be fairly accurately determined from the history alone without necessarily having to examine the eye on a slit-lamp. Important features are:-
i) whether one or both eyes are affected
ii) the presence or absence of any other symptoms such as blurring of vision, light sensitivity, discharge, itching, foreign body sensation or pain
iii) a history of trauma or contact lens wear.
Armed with this knowledge, your GP can get a good idea of the probable diagnosis.
Conjunctivitis usually affects both eyes (but there are always exceptions!) and is associated with a gritty foreign body sensation, discharge and mucky eyelids on waking in the morning.
Iritis (uveitis) is associated with some blurring of vision and light sensitivity (photophobia). Usually only one eye will be affected.
Episcleritis usually occurs in young women and can cause a slight foreign body sensation, but no loss of vision.
Scleritis causes a deep boring ache disturbing sleep.