表面: a physical surface and the limits of appearance
Compare polishing a board's surface with a statement that the outside alone does not reveal the contents. Use the target and verb to read the physical surface and the limits of outward information.
The physical surface being worked on
表面（ひょうめん） means surface or outside. 「板の表面を滑らかに磨いた。」 describes polishing a board's surface smooth. 板の表面 identifies the area worked on, and 磨いた reports the action applied to that surface.
滑らかに means smoothly and describes the resulting surface. 表面 here is a physical surface of the board. 磨いた is past, so read the polishing as an action that occurred without adding a material type or measured values.
What the outside alone cannot tell you
「表面だけでは中身は分からない。」 states that the contents cannot be known from the outside alone. 表面 refers to what is outwardly visible and contrasts with 中身, showing the limits of the information available for that judgment.
だけ limits the basis to the outside alone, and 分からない expresses not knowing. This does not say that every piece of outward information is false or that the contents have already been discovered.
Use the target and verb to distinguish the focus
In the board example, read 板 with 磨いた to identify the surface being worked on. In the contents example, read 中身 with 分からない to identify the limit of what outward information reveals. The focus of the same word changes with the sentence.
The outside in the contents example may also be physically visible, so the uses should not be forced into an absolute physical-versus-nonphysical division. Follow the location of the surface and the scope of the information the sentence describes.
Does the contents example say that all information about 表面 is wrong?
Show the answer and why
No. 表面だけでは limits the basis of the judgment: the outside alone does not reveal the contents. In the board example, 表面 is the surface that was actually worked on. Read the target and verb to determine the scope of each statement.