to the trained eye; if you know what to look for
Fixed expression meaning that someone with expertise or a discerning eye can see what others miss. Often used when pointing out subtle quality, hidden meaning, or skill.
To the trained eye, this painting is clearly genuine.
If you know what to look for, you can immediately see the high level of his skill.
A set phrase combining 見る (to see) with the conditional 〜ば form and the generic noun 人, literally 'if a person who sees sees it'. The repetition emphasizes the act of looking with discernment.