The tutorial explains the methodology which can be used to detect a web server. Web server run on port numbers TCP port 80 for HTTP servers and TCP port 443 for HTTPS.
If any of these ports are detected on a system, then there is high probability that it is a web server. A port scanner like nmap cane be used for detecting if the ports are running on the system remotely. A screen shot of nmap , which scans a local server with IP address is shown below.