Wiktionary
tell you the truth
phr. (context idiomatic informal English) (non-gloss definition: Used to positively assert the frank honesty of an associated statement of set of statements); equivalent to "to you tell the truth".
Usage examples of "tell you the truth".
Because there is no other way of guarding oneself from flatterers except letting men understand that to tell you the truth does not offend you.
To tell you the truth, I've only raised a few birds and they're still too young to fight.
To speak to you, Mister Falkenberg, and to tell you the truth, so that you will either run from us or be damned with us for the rest of your life, for we never lie to our own.
To tell you the truth, I have been arguing with myself all afternoon as to what I should do, and whether or not I should approach you.