<<I'm a [word]>>, the cockroach says again.

The manual helpfully pops up a dictionary.

Does the dictionary know what the cockroach is claiming to be?

A cockroach.

...that makes perfect sense.

"It says it's a cockroach."

"Yes, I know. Does it have anything else to say?"

<<I'm a cockroach!>>

"It just says it's a cockroach."

"Nothing else?"

"Not so far."

"Tell it that it's really important that it say something to indicate whether it's a person or not."

... How is he meant to communicate that?

<<[anxiety about something important] [question particle]>>

<<I'm a cockroach!>>

"I can't be sure because my vocabulary is pretty limited, but... It seems to just say 'I am a cockroach.'"

"Well, that's an approach to everything speaking a language... can your book tell me which things I shouldn't eat because they're people?"

"--it's probably best not to eat horses, marine mammals, or trees? But I don't think there's a list... Yet?"

Quest available: Okay So This One Is The Manual's Fault

Learn to look things up in the manual. 

Success: 10xp, ability to look things up in the manual.
Failure: More conversations like this with Wei Wuxian. 
Accept Y/N

"--or actually there might be a list?" And he accepts the quest.

Is there an obvious way to poke the manual to look things up?

To look things up in the manual, you can say "Table of Contents" for the table of contents, "search" to look for a particular term, or "index" to flip through the index. 

"Search: 'sentience'?"

Sentience, philosophical views of
Speech among nonsentient beings
Panpsychism
Sentience, evolution of
Sentience, moral views on
Sentience, history of moral views on
Sentient species on Earth
Magic use and sentience
Speech grammar for referring to sentients
Etiquette for creatures of unknown sentience
The Harkness Test

'Sentient species on Earth' seems like the relevant result here.

This opens up to a lengthy chapter which explains, in summary, that primates, parrots, horses, marine mammals, and trees are people; mammals, birds, and members of some other species such as octopuses are sentient and can be usefully talked to with the Speech but are not as sophisticated as the ones that are people; and literally everything else can be talked to up to and including thermostats and the air, but in most cases they don't have anything very useful to say.  

"Do you just want to not eat things that are definitely people, or anything that can hold a meaningful conversation?"

"If it can hold a meaningful conversation I don't want to eat it."

"That would be mammals, birds, and octopi. --Fruit is probably fine, though trees are people?"