Check the host object definition for the server in question. i.e.
use windows-server ; Inherit default values from a template
host_name <whatever> ; The name we're giving to this host
Then check the template object (nromally in templates.cfg by default) you are using for this server (i.e. from the "use" variable, in the case above it would be "windows-server"). This may or may not refer to a further template, in which case check that also. What you are looking for somewhere within these objects is the variable that looks like
"check-host-alive" is the command that is being used to check the health of the host. It usually refers to a simple ping command and should be defined (again default) in commands.cfg like
# 'check-host-alive' command definition
command_line $USER1$/check_ping -H $HOSTADDRESS$ -w 3000.0,80% -c 5000.0,100% -p 5
If none of this is looking familiar on your installation then please post your host object and appropriate templates for review.