Reckon that’s something else you’ll get the hang of over time. Think you’ll tend to find most words are of Japanese origin. The practice of adopting foreign loanwords is comparatively recent (like, within the last four hundred years), and use of katakana for them even more so. The word “tempura”, for example, comes from the Latin “tempora” via Portuguese traders, but it’s written in Japanese as 天ぷら (i.e. a mix of kanji and hiragana).
Um… don’t let that confuse you, though. Think the short version is “when in doubt, hiragana”.
“Meishi” is usually written in kanji – 名詞. When a word has kanji but you don’t know the kanji, then you write it in hiragana.