Tail the Apache error log:
tail /var/log/apache2/error.log (substitute different path if applicable)
You can grep for the error log files if you don't know where they are:
grep 'ErrorLog' /etc/apache2/*
The path to watchdog error logs (D6):
http://www.example.com/admin/reports/dblog
To clear the watch dog error log, use these MySQL commands:
(Change path to MySQL specific to your server)
"C:\Program Files (x86)\acquia-drupal\mysql\bin\mysql" <Database Name> -h 127.0.0.1 -u root -e "DELETE FROM watchdog;"
"C:\Program Files (x86)\acquia-drupal\mysql\bin\mysql" <Database Name> -h 127.0.0.1 -u root -e "ALTER TABLE watchdog AUTO_INCREMENT = 0;"
It is critical that auto increment be reset if you truncate the watchdog table; not doing so may cause errors when viewing the log from within Drupal.