You could do it with sockets. Every server has an opened udp socket and the client tries to send a packet to them and waits for an acknowledge packet. ANet although only supports one udp socket opened at the same time so this would be slow. But if you access the windows socket functions from Lite-C you could open more sockets at the same time => faster
But I never tested this, so it's possible that this idea won't work good/fast enough.

Checking it through a timestamp in the sql db is also no bad methode at all. So the decision is yours if you think it's worth to try the socket based methode. I never did...