Q:

How can I tell if a は is the particle は and therefore should be pronounced わ?

On page 82 and 83 of Japanese for Busy People II (third edition), there's the following sentence (written mainly in kana with only a small amount of kanji)
ミルズ：ええ。じつはちょっとねつがあるんです。さっき薬を買ってきて飲んだんですが…。（ハクション）
And a definition of じつは as "actually". jisho.org's definition only has notes of "Expression, Adverb", though when I played the sound for it in an app called "Shirabe jisho", it pronounced the は as a わ, presumably because is the particle は.
Apart from playing the sound for a word, is there any way I could have looked up whether the "は" was a particle "は" or not, either from a dictionary or examining the grammar of the sentence?

A:

Only a few words contain "は pronounced wa" as part of their formal spelling, so it's best to memorize them and get used to them.

こんにちは, こんばんは
ならでは
では, それでは, もしくは, あるいは, または
※ 実は, 要は, 実際は, 本当は ...
※ 願わくは, 望むらくは, 惜しむらくは ...

Etymologically, all of these は are thematic-wa.
※: These are used like (sentence) adverbs, but they may or may not be single-word expressions. Some dictionaries seem to treat these as lexicalized 副詞 (adverbs).

