These endings come very natural to me as a native English speaker, but they must be hell for those who are learning English. I spit out "...haven't they?", "...aren't we"?, "...doesn't he?" effortlessly.
In the sentence you provide:
"This is them,
isn't it them?"
I assume that "this" (the subject of the sentence) refers to a group of people. "This" is singular, so the correct way to say it would be:
"This is them, isn't it?"
Though that's not
totally correct, since "is" is a linking verb and thus the object must be a subject pronoun. Hence, the real, grammatically-correct rewrite would be:
"This is they, isn't it?"
But that sounds weird, even though it's correct and what we are taught in school.
All that said, I wouldn't be surprised if lots of English speakers would substitute "...right?" at times when the alternate phrase is overly complex.
"That's them, right?"