Monitor the number of open sockets on your server.
For example:
# ./check_connections -p 443 -w 400 -c 500 -d
established tcp connections (5) is ok|'max'=500;;;;0 'total'=169;;;;0 'established'=5;;;;0 'time_wait'=164;;;;0 'close_wait'=0;;;;0 'fin_wait'=0;;;;
This will monitor the current https connections and throws a warning with 400 and more connectiond and critical with 500 and more. You can also gather performance data if you specify -d.
-h Show this message
-p Port
-w Max established connections (warning)
-c Max established connections (critical)
-d Performance Data
Copy check_connections to your plugin folder and create a service/exec in your monitoring engine.