command for checking Apache configuration
Solution 1:
Check: http://httpd.apache.org/docs/2.2/programs/apachectl.html
apachectl configtest
Solution 2:
Another way is httpd -t. Therefore, it's available in Windows-version of Apache. Check http://httpd.apache.org/docs/2.4/programs/httpd.html
Solution 3:
apachectl configtest is the correct answer.
Unfortunately I've got a windows installation where apachectl is missing.
Here calling httpd also helps.
Solution 4:
The Apache config test (apachectl configtest, or its equivalents) only tests the config file (and the files it recursively includes) for valid syntax. However, the original question asked for preventing downtime. Even when apachectl configtest does not return an error, an actual restart may still fail, causing downtime.
Common causes for such failures include missing or inaccessible SSL certificates, missing directories for log files or a missing website root directory. Often, such errors are caused by removing a vhost's directory without removing the vhost Apache config file. It is highly recommended to use a tool like puppet or ansible to prevent such inconsistencies.
Seeing that this question is the number one hit when googling "apache config lint" I thought I'd mention this little detail...