I haven't been able to find an answer.
2. See if you can convince some other person to give you money to solve a problem, where your solution involves something besides or in addition to physical labor. (Many but not all jobs qualify.)
3. Success? Congratulations, you are very likely not retarded.
3b. Not convinced because of the "many-but-not-all" qualification? Keep repeating 2 until you are.
3c. I admit that this will never refute the idea that every other person in the the entire world is conspiring to make you feel normal in spite of your deficiencies. (And now I'm part of the Conspiracy.) This seem identical to me to the problem of solipsism, so it doesn't bother me to not have a good answer for that.
I'd say that you should seek external validation, i.e. make something and have it ripped apart on HN.
- Stephen Hawking
On the other hand I felt the same of my dancing abilities (and still do) but being praised(+) a lot by lots of people, some of them being international dancing figures, and one of my teachers - whom I find an excellent dancer - asking me to enter into her (not truly professional but important to her) dance company has made wonders to improve my self-image. Even though I feel clumsy from time to time, once a dance or so ;)
So I guess trying to improve more in what you feel you do badly until everyone praises that aspect of you can help it.
(+) I've got to say the praises where not about my technical level, but more about energy, fun and all the things that make a dance alive.
Dude, you've gotta find a better place for that aside...