Test against both an actual post/page (which would theoretically hit page cache, APC, varnish, etc. for every request after the first request), and against a 404 (e.g., randomly generated strings), which would require database queries on each hit.
A few helpful flags:
-bbenchmark (don’t wait between requests)-c Nwhere N is the number of concurrent requests to make-t Nwhere N is the time (e.g.,30S) to run the test
If it’s helpful this script will generate random strings (404s) which can be used to generate requests to posts/pages which will generate 404s.