The "Server" response header will usually tell you but may omit a specific version depending on configuration.

Use "curl -I whatever.com" to send a HEAD request and look at the headers in the response.

You can also try Nmap, but apparently it's not able to tell too much if the server doesn't reply with a header:

  nmap -T5 -sV --version-all -p 80,443 www.google.com
  Starting Nmap 6.00 ( http://nmap.org ) at 2015-04-16 03:02 CEST
  Nmap scan report for www.google.com (80.202.12.244)
  Host is up (0.0015s latency).
  Other addresses for www.google.com (not scanned): 
  (...)
  rDNS record for 80.202.12.244: cache.google.com
  PORT    STATE SERVICE  VERSION
  80/tcp  open  http     Google httpd 2.0 (GFE)
  443/tcp open  ssl/http Google httpd 2.0 (GFE)
  Service Info: OS: Linux; CPE: cpe:/o:linux:kernel

Apparently stackoverflow (well know user of .net stack) is "unknown", but microsoft.com gives:

  nmap -T5 -sV --version-all -p 80,443 microsoft.com

  Starting Nmap 6.00 ( http://nmap.org ) at 2015-04-16 03:05 CEST
  Nmap scan report for microsoft.com (134.170.188.221)
  Host is up (0.18s latency).
  Other addresses for microsoft.com (not scanned): 
    134.170.185.46
  rDNS record for 134.170.188.221:  
    microsoftproductionstudios.org
  PORT    STATE SERVICE  VERSION
  80/tcp  open  http     Microsoft IIS httpd 8.5
  443/tcp open  ssl/http Microsoft IIS httpd 8.5
  Service Info: OS: Windows; CPE: cpe:/o:microsoft:windows

I didn't look to carefully at the so-output -- maybe there's a funky loadbalancer in front or something.