How to Tell if You Have Synesthesia
If you think you might have synesthesia, but aren't sure, first see if you have any of the risk factors for it, which are being female, being left-handed, and having a relative with the same condition. Next, make sure you're not having hallucinations, which are random and fanciful, unlike the symptoms of synesthesia, which are predictable and repeat themselves. To rule out hallucinations or other serious medical conditions, visit your doctor for a diagnosis and possible treatment.
Realize that synesthesia is relatively rare but likely under-diagnosed. Synesthesia is considered a rare neurological condition that affects the senses, but it's likely that many people who have it are either undiagnosed or assume that other perceive the world just like them. It is unknown how many people have synesthesia. Know that not everyone with synesthesia experiences it physically. If you actually see colors in the air, smell, hear, or feel things, you have projected synesthesia. This form of synesthesia is rarer than associated synesthesia and is what people first think of as synesthesia. Some people with synesthesia (called synesthetes) hear, smell, taste or feel pain in color. Others can taste shapes or perceive written letters and words in different colors. For example, they may see an "F" in red and a "P" in yellow when they read. Some synesthetes see abstract concepts, such as abstract shapes, units of time or mathematical equations floating in space outside their bodies — this is termed "conceptual synesthesia." Identify your risk factors for synesthesia. According to research done in the U.S., there are some factors strongly associated with synesthesia. For example, about 3x more women than men have synesthesia in the U.S. People who have synesthesia are also much more likely to be left-handed and have a 40% chance of a relative having the same condition. Don't confuse synesthesia with hallucinations. Often when people talk of their synesthesia, others assume they are hallucinating or on drugs. What distinguishes true synesthesia experiences from hallucinations is that they are repeatable and predictable, not fanciful and random. For example, if you taste strawberries when you hear a certain song, then one must always trigger the other sensation in a predictable fashion to be considered a synesthete. It doesn't always have to be two-way, though. Synesthetes often note being teased and ridiculed (usually starting in childhood) for describing sensory experiences that others can't experience. Be aware that no two people with synesthesia have the same experience. Synesthesia is a sort of cross-wiring of the nerves and brain synapses related to the five senses. And no two synesthetes have the exact same wiring scheme. For example, the most common form of synesthesia is grapheme-color, when numbers and letters each have their own color. The colors assigned to each letter are different for everyone, but many people have red A's. Another common form is chromesthesia, or colored hearing — sounds, music or voices that are heard and also trigger the eyes to see colors. However, someone may see the color red whenever they hear the word "dog", whereas another might see the color orange. Synesthetic perceptions are specific to each person. See your primary care doctor. Because the sensations of synesthesia can mimic certain medical conditions and head injuries, it's a good idea to see your doctor to rule out anything serious. They will check the functioning of your brain, reflexes and senses to see if you have any physical problems or deficits. If they believe that it is something serious, they might refer you to a neurologist. Keep in mind that people with synesthesia typically pass all standard neurological exams and are considered normal in that way. If you have a neurological deficit that's causing visual sensations, then you're very unlikely to also have synesthesia. Head trauma, post-concussion syndrome, brain tumors, brain infections, migraine headaches, seizures with auras, epilepsy, cerebral stroke, toxic reactions, LSD "flashbacks" and experimentation with hallucinogens (peyote, mushrooms) can all produce sensory phenomena similar to synesthesia. Synesthesia is usually present from birth, so developing it as an adult is extremely rare. If it comes on suddenly in adulthood, see your doctor immediately for an assessment because it could be related to a problem with your brain / nervous system. See an eye doctor. Some visual sensations of synesthesia can also mimic certain eye diseases and conditions, so it's a good idea to see an optometrist or an ophthalmologist to get your eyes examined. Eye trauma, glaucoma (pressure in the eye), cataracts, retinal or vitreous detachment, corneal edema, macular degeneration, and optic nerve dysfunction are all eye conditions that can produce visual phenomena and color distortion. The vast majority of people with synesthesia do not suffer from any physical ailments of their eyes. An ophthalmologist (eye disease specialists) is likely a better choice than an optometrist, who mainly focuses on determining the acuity of your eyesight and prescribing glasses/contacts. Understand that some doctors do not believe in synesthesia. You may encounter some doctors who do not believe that the condition exists. Furthermore, some insurance companies may not cover treatment. You should still visit a doctor to rule out any underlying conditions that might be causing your symptoms, but be aware that your doctor might diagnose it as something else entirely. You may want to get a second opinion if you believe that your concerns are not being taken seriously by the doctor. If your doctor says you do not have synesthesia but have a different condition entirely, trust their advice, and follow their instructions for treatment. 
Realize that synesthesia is relatively rare but likely under-diagnosed. Know that not everyone with synesthesia experiences it physically. Identify your risk factors for synesthesia. Don't confuse synesthesia with hallucinations. Be aware that no two people with synesthesia have the same experience. See your primary care doctor. See an eye doctor. Understand that some doctors do not believe in synesthesia. 