Since there is no ‘real’ test you can actually take to know if you have synesthesia (when I say real, I mean, that you’d get your blood analyzed or a brain scan… on to the brain scan later), the diagnosis is pretty hard. But, you can diagnose yourself.
Pinpoint which kind of synesthesia you may have. There might be only one, or there might be a dozen, or even more!
Search on Google or any search engine for any article for each type, and search for some people’s experiences with the type of synesthesia you are looking for.
Compare how you experience them, and remember that it is normal that everything you experience isn’t the same as other people’s experiences, so, OF COURSE it’s going to be a little different.
For some types of Synesthesia, you can take the Synesthesia Battery, which is an online test. If your score is less than 1, you have synesthesia. Now, this is a long test, but you can still save along the way. I recommend taking this, because there was still a little doubt in the back of my mind about my synesthesia, and taking this and having a score .33 took all the doubt away.
Since the Synesthesia Battery test is only for some types of synesthesia, and so maybe the previous step isn’t for you, but make sure your synesthesia is consistent and unintentional. For example, an A for me will always be a pale sandy kind of beige.
Also, this is a well known test for grapheme-color synesthetes (with numbers), you need tofind the twos quickly! But, my twos and fives have the exact same color… yellow. So, I could still see them quickly, but not as quickly as other synesthetes.
Hopefully, this helped you a little. Maybe you discovered that you also are a synesthete? Maybe you already knew that you were a synesthete, and this just confirmed it!
*About the brain scan, there is a way with MITs… I think… but it would cost way too much money, and the results would still be vague*
(If you want to add anything in the comments, don’t be afraid! :D)