How do you know when to read on'yomi or kun'yomi?

Install this userscript KunOn script

If you have never installed a userscript, please read this visual guide on how to install a userscript.

Read the Textfugu explanation of onyomi / kunyomi

Following is my explanation:

Pink kanji for 女 is じょ. Pink kanji items only have a single character. When you learn a pink kanji, WaniKani usually (but not always) gives the on yomi reading (see below). The authors are trying to pick the most commonly used reading.

Purple vocab for 女 is おんな. Purple vocab items contain one or more kanji characters. They may also include hiragana characters. When it is used with other kanji characters, it is usually (but not always) the on yomi reading. When it is used alone or with hiragana, it is (usually?) the kun yomi reading. Sometimes there are two kun yomi readings in a purple vocab item (usually for body parts).

Japanese has multiple readings (spelling and pronunciation) for the same kanji character. It is frustrating but we all have to deal with it.

The readings are grouped into two groups: on yomi and kun yomi.

On yomi means it is the same reading used by the Chinese. Before you say that your Chinese friend has never used these on yomi readings, it’s OLD Chinese. The on yomi reading is often (but not always) used in combination with other kanji characters. 女 → じょ is an on yomi reading.

Kun yomi means it originated in Japan. 女 → おんな is a kun yomi reading. 女の人 (おんなのひと) uses the kun yomi (おんな, ひと) readings because there is hiragana in the phrase. 女王 (queen) uses the on yomi (じょ) reading because it is linked with another kanji.

Remember these are not hard and fast rules. Japanese is full of exceptions.