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?