With UA and Star Alliance, some Star Alliance airlines let you check award inventory without logging into their programs. That will tell you which, if any, AC (for example) flights are available to AC Aeroplan members (similar for other Star Alliance airlines). Those available to UA MileagePlus members will be a subset of those flights, but at least you'll find out before calling which Star Alliance flights are worth asking about when you are on the phone to UA (i.e. don't bother asking about the flights that are not available at the "saver" level to the other airline's own frequent flyers).