On the fence
Undecided or unsure about something.

In two minds
Uncertain or conflicted about a choice.

Doubt one's word
To be skeptical about what someone says.

Cast doubt
To make someone question or feel uncertain.

Leave no stone unturned
To try every possible option to find something or prove a point.

Second guess
To question or doubt a decision after it has been made.

Out of doubt
Without any uncertainty.

A shadow of a doubt
A slight uncertainty.

Raise eyebrows
To cause suspicion or doubt.

At a loss
Uncertain about what to do or say.

Take it with a grain of salt
To not take something too seriously or with skepticism.

A leap of faith
To trust something without proof or certainty.

Penny for your thoughts
Asking someone what they are thinking, often when they seem doubtful.

Have a doubt in mind
To have uncertainty about something.

Question someone's judgment
To doubt the decisions or opinions of someone.

Hedge one's bets
To protect oneself against uncertainty by having multiple options.

Throw doubt on
To make something seem uncertain or questionable.

Seeing is believing
Doubt can be removed only through direct experience.

Doubt the truth
To be skeptical about the authenticity of a statement.

Doubt creeps in
Uncertainty slowly begins to appear.