When talking about golds, an xback is an aro that has gold rim developing/developed on it's 6th scale row, a highback will reach the 5th level, and anything under that is just an RTG (red tail gold). In the first picture you can see gold on the 5th level clearly so immediately you know it isn't a regular RTG, in the second picture when looking right at the top of the fish you can see gold rim developing on the 6th level.
Hope that answers your question.
Heres a picture that I hope helps show the levels to you.