How does he know which member of Black Order went after the time stone?
Well, Thanos, of course, knew that Ebony Maw and Cull Obsidian will go to the Earth to retrieve Time Stone cause he planned it all.
Why didn't Thanos consider Cull Obsidian being dead? Am I missing some plot?
It was never explained in the movie nor by someone from the production team. However, there is an explanation.
By the time Thanos reaches Titan, he has obtained the Soul Stone. This infinity stone lets the one to steal, control, manipulate, and alter living and dead souls. It also allows the one to control all lives in the universe.
Allows the user to steal, control, manipulate, and alter living and dead souls. The Soul Gem is also the gateway to an idyllic pocket universe. At full potential, the Soul Gem grants the user control over all life in the universe.
Since, it allows to control the soul, it is possible that he used the soul stone to take a look who is alive. Ebony Maw gets killed after being thrown into the space, but Cull doesn't. He gets stranded on a remote field when Wong creates a portal. Remember, he is not dead when Thanos reaches Titan.