Before (re)starting nginx, the configuration files should be tested:
service nginx configtest
(The test will also be performed when loading or starting nginx, but it is good practice to check it manually.)
To check the configuration files using nginx itself:
/usr/sbin/nginx -t
The output should look something like this:
nginx: the configuration file /etc/nginx/nginx.conf syntax is ok
nginx: configuration file /etc/nginx/nginx.conf test is successful
Important: nginx does only check if the config file is valid. It does not, for example, check if the PHP-FPM ports or sockets are working.
Remember that /etc/nginx/nginx.conf has a reference to include the configuration files for each enabled website (/etc/nginx/sites-enabled/*). If an error is found in one of these files, it will show in the output.