When I look at Chinese vs Japanese calligraphy samples and books, it seems to me that there are subtly different aesthetic principles governing the characters, even discounting variants (including stroke order switches etc) and different formal styles. For argument's sake, let's limit the argument to 楷書 where things would be expected to be the same. Can an expert (or even a knowledgeable layman) see a sample of 楷書 and identify it as Japanese or Chinese? Or are the variations between calligraphers wide enough that national identity does not stand out as a distinguishing factor?
On a per-character basis, generally not. Calligraphic styles are relatively standardized across the Sinosphere. The only real exceptions to this are where distinctly nationalistic elements appear when looking at the broader text as a whole:
- Simplified Characters, in the case of Chinese
- Hangul, in the case of Korea
- Kana and certain Japanese simplifications of characters, in the case of Japan
It's customary in both styles of calligraphy to use the traditional forms of characters, so simplified forms don't always provide an indication. That said, many simplified characters are derived from calligraphic forms (e.g. 门 vs. 門), so these are a weak indicator at best.
The other distinguishing meta-factor is the signature on the piece, as naming conventions are different across each country.