How would I know if the bus 919 I'd see will go to Beijing Zoo or not?

No. The bus no. 919 does not go to Beijng zoo.

Hello Kate. Thanks for your reply.

I found the following above that's why I asked the question. It gave me the impression that there're buses with 919 labels which go to Beijing Zoo.

"After your Badaling hiking, catch the bus or train as you did in the morning. When you wait for the Bus No. 919, you may find the bus running out of the station full seated. Don't get on this kind of buses. They are running to Badaling Zoo instead of going back to the downtown city."

There are several buses with 919 labels which go to different directions. The zoo you mentioned means Badaling Satari World or zoo in downtown area? The bus no. 919 can not take you to the one in downtown, but if you go to Badaling zoo, you can take Bus No. 919 Nonstop Express (Yan Qing) from Badaling. To avoid any misunderstanding, you are suggested to consult the staff there.