A computer that is conected to a LAN or Internet usually offers some services. If we talk about a normal network that uses TCP/IP protocol, each of the services is going to use a port. For a PC to use a service, it needs to specify the number of the port.
The first step to know if a remote machine is running a service is testing connectivity. There a lot of programs you can use for this but today I am going to use PortQryUI tool. This utility can be downloaded from Microsoft:
PortQryUI Tool. Microsoft Download Center.
I have selected the application because is very simple to use. If you want to use something more professional try NMAP; you can use Google to find it.