Honesty is the best policy.
Being honest is the most effective way to achieve good outcomes.

The truth will out.
The truth will eventually be revealed, no matter how much one tries to hide it.

A clear conscience.
Feeling no guilt because one has acted honestly.

To come clean.
To admit the truth about something.

To spill the beans.
To reveal a secret or disclose information.

To speak the truth.
To tell the truth without lying.

To lay one's cards on the table.
To be open and honest about one's thoughts or intentions.

To be above board.
To be honest and transparent in one's dealings.

No ifs, ands, or buts.
No excuses; being straightforward and honest.

To call a spade a spade.
To speak honestly and directly about something.

To face the music.
To accept the consequences of one’s actions honestly.

Honesty pays off.
Being honest leads to positive results in the long run.

To tell it like it is.
To speak frankly and truthfully.

What’s done is done.
Accepting the reality of a situation honestly without regret.

To have nothing to hide.
To be completely honest with no secrets.

Truth is stranger than fiction.
Real-life events can be more surprising than made-up stories.

To keep it real.
To remain honest and genuine in one's actions or words.

To own up to something.
To admit to a mistake or wrongdoing.

To give a straight answer.
To respond honestly without evasion.

To shoot straight from the hip.
To speak honestly and bluntly without hesitation.