All API calls for management interface are referring to MX, even the one you said, from v0.
Therefore, I don't think there is a way to determine the Uplink for a MS switch, unless you prepare it yourself.
For instance, you can tag the uplink ports so later you can identify them via tag.
Or, another way would be to check the CDP/LLDP neighbour and try to compute it yourself, if your network IP address scheme permits.