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Here’s a simple function using getaddrinfo() that will take an IP address and return the address family (AF_INET for IPv4, AF_INET6 for IPv6, etc). I works on both Linux and Windows. This function will also accept hostnames and return the address family of the first address returned. You can disable this feature (and the corresponding DNS lookup) by passing the AI_NUMERICHOST flag.

// Returns the address family of an address or hostname.
// AF_INET, AF_INET6, or -1 on error.
int getaddrfamily(const char *addr)
{
    struct addrinfo hint, *info =0;
    memset(&hint, 0, sizeof(hint));
    hint.ai_family = AF_UNSPEC;
    // Uncomment this to disable DNS lookup
    //hint.ai_flags = AI_NUMERICHOST;
    int ret = getaddrinfo(addr, 0, &hint, &info);
    if (ret)
        return -1;
    int result = info->ai_family;
    freeaddrinfo(info);
    return result;
}

See RFC 3493 for more information on the latest socket API for dealing with IPv6.