You first need to find your clan affiliation. Best way to start is type in a search engine..like google " clan sept (and your last name)" IE clan sept keith or clan sept smith etc.. that should give you a good start to find what clan you belong too.

You could also look at http://www.scotclans.com/whats_my_clan/
you can type in your name and it will hopefully provide and answer

You can try you last name or in some cases try a name from the maternal side of the family.

Once you find a clan.. check the official clan site and see if they recognize that as being a SEPT (sub-family) of the clan