I think I have a vivid imagination. So many of these things I can say “I know what you mean!” about, but I don’t know if I really do or I just imagine that I could. Definitely seems to be in my brain rather than actual perception…
Well, there are two kinds of synesthesia: associative and projected. Associative is actually quite a bit more common, meaning that the majority of synesthetes do not actually SEE colours and shapes floating before their eyes or superimposed on the letters and numbers they read, but rather their minds persistently associate a colour or texture or smell or taste with certain graphemes or sounds.
Real synesthesia is consistent over time, however, and doesn’t require concentrated thought because the associations are ingrained and automatic. So if you have to ponder what personality or gender or colour or taste the number 3 might have, then you’re probably not a synesthete. But if someone asks “What direction is Tuesday?” and without stopping to think about it you immediately point up and to your left (or whatever), you likely do have synesthesia.
Those are just examples, of course. There are many different kinds of synesthesia, and no two synesthetes’ impressions or associations are exactly alike. For more information, I highly recommend checking out the excellent Tumblr a-synesthetic-world!