Although not that much useful, I think it is better to add "read receipts" to answers and comments (to notify the answerers/commentors about whether their answer/comment was read by the OP).
Although not that much useful
Indeed, I don't think this is a very useful feature. I don't see why you would actually want this. It will only cause users to complain "you read my comment, but didn't answer!". Responding to a comment is not required, and we shouldn't make any suggestions it is.
If you really want to know if a user most likely read the comment, you can see the time the user was last online. For me, that is a good indicator if the user has read the comment or not.
I think it would increase animosity a tad by knowing that you were 'seen' but not 'heard' when answering a question. It should be safe to assume that the OP is probably standing by with bated breath looking for a solution.