A esp32 check to see if any network interface up.

esp32_netif_isup.ino

#include <tcpip_adapter.h>

static bool netif_isup() {
    for (int i = TCPIP_ADAPTER_IF_STA; i < TCPIP_ADAPTER_IF_MAX; i++)
        if (tcpip_adapter_is_netif_up((tcpip_adapter_if_t)i)) return true;
    return false;
}