You might try Google Flights; you might also try a multi-city search on United. Figure out whether you'd rather transit PEK on your way to or from Tokyo and search accordingly. The fare is still in place, and it allows you to fly either ANA or Air China from Tokyo to Beijing.

Today, however, I'm only seeing ANA flights on United.com -- Air China is missing -- which is reducing the options considerably (BOS-(EWR/ORD)-PEK-NRT would require an overnight in PEK; HND-PEK-(EWR)-BOS works, although the 3 hour connection is a little tight). You should still be able to get Air China flights instead; you just might need to call UA if Google Flights doesn't help. The fare is available for outbound travel through this Saturday or from Christmas through the end of April.