Upon further consideration, I'd like to add the following. It seems to me that inflection (or not) could indicate the use of the question mark rather than the period.
Mark's a slob isn't he! (I'm sure you agree with me....not really asking.)
Mark's coming tomorrow night, isn't he? (I have some doubt, so I'm asking.)
The same with the negative.
Mark isn't a slob, is he! (Someone said he was, and I disagree; I want you to say I'm right.)
Mark isn't coming tomorrow night, is he! (I thought he was going to, but I suspect he isn't, so i want confirmation - might indicate disappointment on my part.) Mark isn't coming tomorrow night, is he? (might also indicate disgust).