On account of the complex symbiosis between Finnish- and Swedish-speaking people in the bilingual regions of Finland, you cannot always immediately tell whether someone is a native-speaker of Finnish or Swedish, or what their second-language is likely to be. There are relatively few true bilinguals; Finnish-speaking people have picked holes in the Finnish of such people as Elmer Diktonius and Jörn Donner, even though they seem incredibly bilingual to the outsider. Whether this criticism is accurate, or mere envy, I cannot tell.
But it is the Christian name, or forename, that often (with some exceptions) gives a clue to the main language of specific people from Finland. Take a number of authors, statesmen and cultural figures in no particular order. Some will fool you, others not, and some speak somewhat unexpected languages:
Arvid and Eero Järnefelt
Johan V. Snellman
All names ending in -ius
Hannele Mikaela Taivassalo
Heidi von Wright
And so on. I'll continue later, in a comment on this same thread.