You will need to check-in with the partner. Depending on the partner, they may or may not be able to check you in downline for the UA flight. If not, you will need to obtain your UA boarding pass at the transfer desk or at the gate.
As others had pointed out, the partner's record locator may be available through United.com. If not, you can obtain the locator through the partner by providing date, flight information, and name.