If your configuration does not include a root /some/absolute/path; statement, or it includes one that uses a relative path like root some/relative/path;, then the resulting path depends on compile-time options.
Probably the only case that would allow you to make an educated guess as to what this means for you would be, if you downloaded and compiled the source yourself. In that case, the paths would be relative to whatever --prefix was used. If you didn't change it, it defaults to /usr/local/nginx. You can find the parameters nginx was compiled with via nginx -V, it lists --prefix as the first one.
Since the root directive defaults to html, this would, of course, result in /usr/local/nginx/html being the answer to your question.
However, if you installed nginx in any other way, all bets are off. Your distribution might use entirely different default paths. Learning to figure out what kind of defaults your distribution of choice uses for things is another task entirely.