It's pronounced "wa" only when used as the topic marker, for example ”あなたは" ("as for you", which means "I'm about to say something that has you as the topic") or "こんばんは" ("good evening" but literally "as for this evening", although it's probably uncommon to literally mean "I'm about to say something that has this evening as the topic"). All other times it's pronounced "ha".

は如果是 “助词” 的话，就发 wa 其他情况都是发 ha 的

It's easy to distinguish. After subject we pronounce "wa". Others are pronounced "ha"