Filename has "soybooru" word (example: "111111 - SoyBooru.png" or "SoyBooru.com_-_111111_-_tag1_tag2_tag3...") or filename ends with a letter (example: "1761730873366z.png")
They post soyjacks (of course)
They're obsessed with nigger dicks and "clitties"
They quote other people with soyjacks or other shit

They use their shitty niggerbabble slang aka "soyspeak":
>geg (soicuck version of "kek")
>'p
>gem
>coal
>ack
>clitty
>oyyyyyy doctos
>trvthnvke
>trvke
>BBC therapy
>cado