how I can detect if the device is connected?

AFAICT, there's no way but I'd like to be wrong. In a protocol I had to implement, there were two messages designed to detect this, the typical ping and pong. On startup, a ping was sent and the program assumed there was no connection if no pong was received. Both peers did the same. So, in any case, the second machine that went online caused the connection to be active.