If the mjpg-streamer is running as daemon, its output is not visible. So, to keep track of the daemon, it should print at least fatal messages at startup to the syslog.
Something like the following would be useful:
* no plugins found
* could not open input device
* running httpd now on port 8080
The included webserver should also log some details about served requests. Perhaps this can be logged to another file at /var/log/....