I hope this helps:
Two rules of thumb for it almost certainly being kun:
If it’s 3 or more morae (“syllables”), it’s kun. 美しい => うつく
If it’s 2 morae and the second isn’t つ、ち、く、き、ん then it’s kun. 人 => ひと=> kun.
BUT the above don’t apply, it could be either. 立 has two main readings which have two morae and DO end in つ: リツ (on) or たつ (kun). You just have to learn until that intuition kicks in.
Onyomi is derived from Chinese readings borrowed at different points in history.
Chinese and Japanese native sounds don’t map perfectly - in particular, Japanese morae always end with a vowel (ん takes up a separate beat).
Chinese Hanzi only ever have one syllable - but they do end in -ng and used to end in k, t, p and m as well (in the Cantonese dialect, still do). So as Japanese morae can’t end with k or t, an extra morae of き、く、ち、つ would be added. 国 => コク because the pronunciation of the Hanzi, when the kanji was borrowed, was “kwok”.