By using the code from https://github.com/takeshixx/ip-https-tools/blob/master/ip-https-discover.nse as a base. The following health check for a load Balancer will determine if Direct Access is availability and accepting new connections
Our our F5, we have a HTTP Monitor that is sending the string
GET / HTTP/1.1\r\nHOST: <DADNSNAME>\r\nContent-Length: 18446744073709551615\r\nConnection: Close\r\n\r\n
Replace <DADNSNAME> with the FQDN of your DA implementation
The receive string is
HTTP/1.1 * (200)