There are some very generic guidelines, e.g. "kanji compounds (words with two or more kanji) usually use on'yomi", but no way of knowing for sure. Some compounds use kun'yomi or a mix of the two, and there are words like 大人 ("adult"), which don't use either on'yomi or kun'yomi (it is read おとな). If you look at the second link in my previous post, you can see they list a common example for 一 、 the word 一日 which is read ついたち (when referring to dates, like "April the 1st").
That is why it is important to learn words, including these common exceptions. Eventually you'll learn to make good guesses based on words you already know, but even native speakers can get caught out by difficult/unusual readings. One minor 'trick' that can sometimes help with on'yomi: many kanji have phonetic markers in them. e.g. 長、腸、張 all have the on'yomi チョウ , and 同、銅、胴 all have the on'yomi ドウ. In these cases the left hand side of the kanji has to do with meaning and the right hand side with sound.