truly vs in truth

truly and in truth both are adverbs.

Word	Noun	Adjective	Verb	Adverb
truly	No	No	No	Yes
in truth	No	No	No	Yes

As adverbs, truly and in truth are synonyms defined as:

truly and in truth: in fact (used as intensifiers or sentence modifiers)

Other synonyms of truly include really.

truly (adverb)	in truth (adverb)
in accordance with truth or fact or reality	in fact (used as intensifiers or sentence modifiers)
by right
with sincerity; without pretense
in fact (used as intensifiers or sentence modifiers)