best way to debug nginx rewrite rules in config file?

Enable rewrite_log:

rewrite_log on;

and set debug level in error_log directive:

error_log /var/log/nginx/localhost.error_log notice;

Enable debugging support, then set debug level in error_log.

error_log   /var/log/nginx/error.log debug;

Now you can tail the log and send your requests through. There's probably more detail than you want, but that can sometimes be a lifesaver.

Oh, and you should be aware that if is evil, in a location context at least...

Using logs and and built-in support for debugging is definitely the most reasonable way. If you are doing some quick routing debugging at early stages and want to interact through the browser/client only, using the return 4xx "text"; directive may also give you the answer you want with very little effort. For example,

http {
  server {
    listen 80;
    server_name mydomain.net;
    return 404 "mydomain 80 route";
  }

  server {
    listen 80 default_server;
    return 404 "default 80 route";
  }
}

The text in the returned webpage will tell you which server block your request triggered.

Another good debug step is to check if your Nginx rules are being taken into account at all:

location /test {
    return 200 'My test';
}