[jey]

By studying the code?

If all you noticed from seriously looking through the source is that it is well indented and well organized, you're probably not an experienced enough C++ programmer to evaluate the merits (no offense intended).

[capablanca]

Indeed... So by clean you mean looking for usages of certain C++ features that should be avoided?

How can you "infer" the design only by looking at a pile of C++?

[pohl]

One probably couldn't by merely looking at it. Reading it with the specific intent to understand it, on the other hand, can make design intent pretty clear. (One would hope that the structure of the program is at least homomorphic to the intended design, if not isomorphic.)