The title says it all: is it possible with a procedural macro (derive) on a structure to check whether the structure is repr(C)?
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Macros don't have any information other than the literal source code you give them. They run before any types are known, so there's no information about repr(C). A macro (derive or attribute) on the type would be able to check for the repr(C) (and repr(transparent)) attribute, though. There may be tricks for sharing info between macros, but I haven't looked into it. The only similar case I needed was for deriving a trait that would also imply repr(C) or repr(transparent).
Apparently, it's trivial:
That requires the macro to be on the structure and I thought you didn't want that. 
But if that's fine, then it's about as easy as it looks.
I think you had a very unfortunate typo here, saying without 
Oh man. I'm so sorry.
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No worries, it just made the answer more indirect 