Honestly, if you fear you have it, and you have health insurance of any sort - go to your general practitioner. Then tell them you are feeling tired all the time. They will present you with The Epworth Sleepiness Scale. Even if you don't score high, but have continuous sleepiness for a long period of time, you will likely get a referral to a sleep specialist.

If you are sleepy no matter how much sleep you get (as in, you can sleep 8-10 hours and still be sleepy), then you probably have some sort of sleep disorder, OR are narcoleptic.

Assuming you progress to a sleep study, they will have you stay overnight and into the next day and monitor your sleep. The night-time stuff is where they screen for things that disturb sleep (apnea, etc). The next day, they do maintenance of wakefulness and other tests to see if the issue might be related to narcolepsy. The way they find out here is if, when they offer you nap periods, you fall asleep too quickly, and enter into the REM stage too quickly - this is a strong sign of narcolepsy.

There are different forms of narcolepsy, not just the one with cataplexy or the "cartoon" type narcolepsy. You may have it.

The good news is, if you do have it, provigil just went generic, and nuvigil works very well (it is more expensive, but you might get a discount card, and it helps if you have good insurance). There are a good number of low-grade narcoleptics that have been diagnosed with ADD (not ADHD) due to inability to focus - it is a very common symptom.

Edit: Funny thing about the ADD diagnosis - things like adderall (read: amphetamine) worked just fine with people with narcolepsy, because that was the drug class used before provigil came out to treat it. It just has more side-effects.