Simple, do a GET or HEAD HTTP request to the monitored website with a 3 second timeout. If you get a 200 response code then you can assume the website is online and okay.

Why HEAD? Because:

if a URL might produce a large download, a HEAD request could read its Content-Length header to check the filesize without actually downloading the file. (...) A response to a HEAD method should not have a body

Using HEAD instead of GET will make it so your code doesn't have to actually download your frontpage to get the status. This will speed things up and reduce bandwidth usage.

Note: webservers may also return response codes for redirects, like 301 or 308 and and this case you usually do a follow up request to the URL the server pointed you at in order to check if it returns 200. Some HTTP libraries have built in ways to handling this and with a simple boolean they'll follow the redirect for you.