If you must use sarcasm, here are a couple of common ways:
1) Yeah...because>(rest of comment)
[the ellipsis will clarify that the comment is sarcasm at least some of the time but its reliability is not perfect]
2) end your comment with '/s'
[most people know this means the previous comment was meant as sarcasm. it should be the preferred method of communicating sarcasm and the sentiment that the referenced recipient is obviously wrong]
2) end your comment with '/s' [most people know this means the previous comment was meant as sarcasm. it should be the preferred method of communicating sarcasm and the sentiment that the referenced recipient is obviously wrong]