How to run a manual check from nagios slave

You may need to su to the nagios user to run some checks
sudo su - nagios

/usr/local/nagios/libexec/check_http -H servername.domain.ie -S -w 5 -c 10
/usr/local/nagios/libexec/check_http -I 192.168.1.50 -S -w 5 -c 10
/usr/local/nagios/libexec/check_http -H 192.168.1.50 -S -w 5 -c 10

more info here http://nagiosplugins.org/man/check_http