Truth

Actions speak louder than words.
Appearances are often deceiving.
    Similar proverb: You can’t judge a book by its cover.
Believe nothing of what you hear, and only half of what you see.
    Opposite proverb: Seeing is believing.
The devil can quote Scripture for his own ends.
Honesty is the best policy.
You can’t judge a book by its cover.
    Similar proverb: Appearances are often deceiving.
Fact is stranger than fiction.
Fool me once, shame on you; fool me twice, shame on me.
Through rose-colored glasses the world is always bright.
    Similar proverb: Everything looks rosy through rose-colored glasses. (Prolman)
All that glitters is not gold.
Half the truth is often a whole lie.
A lie can go around the world and back while the truth is lacing up its boots.
Seeing is believing.
    Opposite proverb: Believe nothing of what you hear, and only half of what you see.
Where there’s smoke, there’s fire.
    Related concept: Tip of the iceberg.
If something sounds too good to be true, it probably is.

SAYING
If it looks like a duck, walks like a duck, and quacks like a duck, it probably is a duck.

CONCEPT
Tip of the iceberg