I have made the transfer in PEK before. If you are on an award ticket and changing from UA to Thai airlines you may not have a Thai boarding pass until you get to PEK and check in. On one occasion i had to exit immigration and re enter and on one occasion they let me use the Xfer line even though I had no boarding pass. What I did have was my complete itinarary printed on UA boarding pass stock. I would highly recommend when you check in in the US ask the agent to print your itinarary on boarding pass stock. It is just much more "official" looking when transiting. Good luck