With the grep command.
If you want to find out if the term user is in /etc/httpd/conf/httpd.conf, you would run
grep -i user /etc/httpd/conf/httpd.conf
(-i makes the search case-insensitive).
Normally grep does not tell you the line number where it finds the term. If you want to find out the line number, run
grep -n -i user /etc/httpd/conf/httpd.conf