In the middle of reading a pile of psychophysics stuff I came across an interesting book called "The Man Who Tasted Shapes" by Richard E. Cytowic, who has carried out extensive experiments into synesthesia. In this book he lists 5 features for the diagnosis of synesthesia:

1) Synesthesia is involuntary - it cannot be willfully suppressed or conjured at will, it has an external stimulus, but not everything will casue a reaction.
2) Synesthesia is projected - it is usually perceived outside the body rather than "in the mind's eye".
3) Synesthesic perceptions are durable, discrete and generic - ie. the associations endure for a lifetime - if a certain sound is heard as blue, that will never change; given choices, synesthetes pick only one from a selection; they are unembellished percepts (never complex scenes).
4) Synesthesia is memorable - the parallel sensations are easily and vividly remembered and are often used as a mnemonic aid.
5) Synesthesia is emotional and noetic - synesthetes have an unshakable conviction that their perceptions are real.

The book is a popular science, (self-described "medical detective") story, but is a fascinating read (and sure as hell beat reading dry papers on threshold vision responses).

Also, there was a radio program on synesthesia a while ago where a woman described how she and her aunt both "heard" colours, but her colours were vivid and bright, whereas her aunt's were pastels!