Mensa doesn't accept SAT tests anymore. I think SAT tests are more of an aptitude test and IQ tests are supposed to measure intelligence, although obviously the two concepts are closely linked.
"Not fitting in" doesn't necessarily mean you are or aren't gifted. I know lots of of gifted people/kids who fit in, and some who didn't. Taking the mensa IQ test won't necessarily answer questions that you have about yourself- what if you bomb it? You can still be gifted.