In general, you can judge the accuracy/quality of a translation using a number of factors:
- How old is the translation? Older translations are more likely to contain mistakes for a number of reasons, while newer translations are more likely to reflect a modern scholarly understanding of the text. Translations on the internet are usually older, because really old texts become public domain and can be posted on the internet without legal consequences.
- Who is the translation marketed to? For example, some translations of myths are designed for children, which makes them less accurate.
- What do academics say about the translation? If you look on a site like JStor, you can sometimes find reviews by academics of other books, including translations. These reviews are brief (1-2 pages), and will point out any major problems with the text. You can usually get access to JStor through a local library.
You should know that serious scholars of texts like the Kalevala read the Kalevala in the language it was originally written in (the same goes for something like Greek Mythology, and even for the Bible). All translations contain inaccuracies, and no translation is capable to rendering all of the puns, rhythm, and linguistic features of a foreign text. However, unless you are a serious scholar, a modern translation of the Kalevala should be good enough for your needs.
Regarding the Kalevala, I found this well-researched article that discusses the various translations of the Kalevala. It doesn't recommend any specific translations, but it does talk about the motivations behind translating the Kalevala. You might also be interested in this scholarly article about translating the Kalevala.
You asked for a specific translation, so let me say that this translation by Keith Bosley appears to be a accurate and entertaining translation. I can't say that it's the "best" translation, in part because that is a very subjective question, but it should be good enough for what you are trying to accomplish.