idiom
the proof is in the pudding
本当の価値は使ってみて初めてわかる
Meaning
the real value of something can only be judged when it's put to use
Example
He claims the app is great, but **the proof is in the pudding**.
彼はそのアプリが素晴らしいと言うが、証拠は使ってみてわかる。