The general rule is: if there's a kun'yomi that fits, use the kun'yomi. Otherwise, use the on'yomi. Most commonly, multiple kanji next to each other are on'yom

The general rule is: if there's a kun'yomi that fits, use the kun'yomi. Otherwise, use the on'yomi. Most commonly, multiple kanji next to each other are on'yomi and single kanji are kun'yomi.