Admittedly I haven't flown United in a couple of years, but my experience with them was the same as with every other airline: when booked on a codeshare (i.e.,

Admittedly I haven't flown United in a couple of years, but my experience with them was the same as with every other airline: when booked on a codeshare (i.e., Airline A issues the ticket, but Airline B operates the flight), the itinerary emailed to me would always say "Check in at the (insert operating airline) ticket counter".