I can suggest a little trick.
The one web page implies this idea, though. Just switch the two words at the beginning of the question/phrase (i.e. the contracted interrogative fragment) and if it makes sense, then it's correct form or usage.
E.g. I’m going with you on vacation, am I not? (so do the switch: "I am not" - *I am). Therefore, it's correct.
English speakers are always trying to make shortcuts in speech and although, it's an attempt at efficiency (to say the same thing in less words), it is often considered incorrect grammar. People are also using slang and eventually get into bad habits and form. But, it sounds 'better' ('difficult to explain).