While watching subbed anime to try and understand what is being said, I noticed that instead of saying "desu yo" they say "deyo" in it's place most of the time.
I'm guessing that's slang.
Just now, Some dude in the anime was asking like a bunch of questions but instead of saying "ka" or "desu ka" at the end of his questions, he just simply said "doo" (doe).
Is this observation wrong? If not, is this question slang?