In 2012, I wrote an article about commonly asked questions pertaining to synesthesia. The article can be read in full here
and is also in the Articles topic in the Resources forum. I'll summarize some of the main points below, however.
Do I have synesthesia?
Key features of synesthesia are involuntariness and consistency. If you are having a synesthetic response (for example, seeing E as yellow) that happens automatically (that is, without you having to think about it) and consistently (the same every time), then chances are, yes, you have synesthesia.
How many types can I have?
It's common for synesthetes to have more than one type. Brock, for example, has five.
What is this "projected" and "associated" stuff I've been hearing about?
Projected synesthesia is synesthesia that you experience in the outside world. For example, you may perceive the colors and shapes to a song as somewhere in front of you, that you could touch if you just reached your hand out. Someone with this type of synesthesia is called a projector.
Associated synesthesia is synesthesia that you experience in your mind's eye. For example, you may perceive the color of a letter inside your mind but you won't physically see it appear on the page in front of you. Someone with this type of synesthesia is called an associator.
It is possible for some of your syn types to be projected and others associated.
How can I test for synesthesia?
In the Sites and Tests topic in the Resources forum, there is a link to the Synesthesia Battery. This test is a good way to gauge your own synesthesia.
Tests you can do on your own include writing down or drawing responses you have, putting the list or image away for a while (ex. a month), then writing a new list or drawing a new picture and comparing it to the old one.
Can I gain or lose synesthesia?
Gaining synesthesia is not possible, though in rare cases head trauma can cause it.
Losing it is possible after emotional stress and will often come back once the stressors are gone. It is possible that head trauma can take it away temporarily or permanently.
Can my responses change?
Sometimes, but by and large they remain the same. When first figuring out a new synesthetic response or type, it may be difficult to determine precisely what response you are having to a stimulus. In these cases, it may seem to change until you can pin down exactly what you're experiencing. Once you figure it out, however, the response is unlikely to change.
Can you turn synesthesia off and on?
No. Synesthesia is involuntary. That said, synesthetes may learn to tune responses out or pay more attention to them. Additional, stimulants (such as caffeine) and depressants (such as alcohol) may enhance or dampen responses.
What's the science behind synesthesia? Is it a disease I can catch or a disability? How does one get it?
Synesthesia is a trait, just like blue eyes or brown hair or being six feet tall. It is not a disease or disability. Current research is finding that it may have genetic links as it can run in families.
How many people have it?
Some of the most recent figures state approximately 1 out of every 23 people has synesthesia. It is much more common than originally believed.
I've heard all these stereotypes about synesthetes being artistic, left-handed, female, bad at math and directions...
An artistic, left-handed, female synesthete exists somewhere in the world, I'm sure. However, not all synesthetes are like this. There are male synesthetes, right-handed ones, ones who are good at math or bad at spelling, who can give directions like no other, and so on.
Are synesthetes making it up for attention? It all sounds too strange to be true.
No, they are not. Synesthesia is a real trait with plenty of scientific support behind it. For more, please see the links in the Resources forum and research by Dr. Richard Cytowic and colleagues. Please be aware that telling a synesthete they are just making it up can result in very insecure, self-conscious feelings. Many may have been ridiculed as children when they mentioned it and so kept quite about their synesthesia until now. Others may just be discovering it and in need of support. Please do not take what they say lightly or assume they are just attention-hogs.