A quick way of determining this with Finnish is if it is 1) a nature word + 2) follows a simple phonetic structure where each syllable is CV or CVV (C = consonant V = vowel) then it might be from proto-Uralic and so not a result of language contact but a "really old" Finnish word and may be congnate with words in other Uralic languages e.g. Hungarian or one of several tiny languages Russia. Also e.g. puu (= tree), vuori (= mountain) is proto-Uralic. Other nature words like järvi (= lake) which don't follow the pattern might be proto-Finnic but have come from neighbours (for järvi it is apparently from baltic/proto-slavic).