It goes along a little bit with joke comments, if you follow. I can't for-sure know what the motivations behind any given post are (surely some motivation is to receive karma even on good comments), but strong warning signs that a comment was posted mostly to receive karma include, roughly in order of strong indicators to weak indicators:

• comment is very short
• comment includes an emoticon
• comment is intended as humor
• comment that expresses reasons for having a belief that everyone at Less Wrong already has
• comment has little actual content compared to number of words
• (related to previous point) comment has no content except agreeing/disagreeing
• comment has signals of pseudo-modesty such as, "perhaps", "maybe", "I think", "it seems", "possibly", etc.
• comment is pure speculation
• comment is made after previous ones, where an edit to a previous comment would have been appropriate (users cannot upvote a single comment multiple times, but multiple comments by a single author are fair play)
• comment mentions karma
• comment speaks in passive voice

Multiple items on this list prime me for down-voting behavior.