Its a bit of a judgement call. Probably the easiest way to avoid keyword stuffing is by submitting content that is written in a way similar to how you would talk and share information. Does it make sense or is the article loaded with the key word(s) to the point where it no longer makes sense? Example if say the key word is hamburger, are you mentioning hamburger 17 times in 50 words? That's keyword stuffing! However, if you mentioned in the first sentence and then midway through the paragraph, it's not an issue if it makes sense. It's considered okay by Google and other watchdogs to use a keyword several times in an article, just not in every sentence or randomly in a way that doesn't make sense.