I did an online test on Mensa Sweden's website, and then went to do the real test for a comparison. And the results were utterly consistent. I don't think Mensa as an organisation has an interest in tricking people to do their tests. Furthermore, the validity on online/home tests provided by Mensa seems fairly high, based on my own experiences, and that of a few friends.
Try some internet tests (that don't look completely unprofessional), and compare the results, and you might get a better picture of your chances of doing well on the real Mensa test. But I concur with an earlier post that IQ tests are not a fair way of measuring how "smart" or "intelligent" a person is. It is a highly reductionistic measure of how well (or rather 'desirable') you organize information in a specific fashion, under specific circumstances.