I had this same issue when I started getting into MBTI. I couldn't tell if I was an INTJ or ISTJ. I grew up working on computers, so I was surrounded by NTs everywhere. I would be surprised if my habits weren't NT-like. Part of the problem of figuring it out is easily confusing other things for being an N. Extreme introversion can be confused for being intuitive, because people equate inuitive at times with aloofness. I can easily get lost in my head, but does that necessarily mean I'm lost in abstract thought? No.
You can easily confuse ISTJs also because of Ne. However, you have to realize that Ne is intuition focused outward and Ni is focused inward. Ni is more conceptual in nature, and it focuses on ideas and theories concerning systems of thought. Ne is intuition of the extraverted kind, which means that it's focused on external possibilities. Ni asks, "How can I make an effective hot dog and stand what will it require?" Ne asks, "What potential income does this hot dog stand have in terms of customers coming over a certain time period?"
Being an NTJ and STJ means that your life is also structured. You plan things out. Ni makes a plan, but draws out a vague plan of what is necessary to be accomplished. Si looks at everything necessary to be accomplished, the requirements of each one, and how to plan it out sequentially. For example, an NTJ will say, "I need to go pay the phone bill, go to the gym and buy a present for a friend tomorrow." An STJ will say, "I'll go to the gym in the morning. I can pay the phone bill before have lunch at 12 PM, then after that, I can go shopping in the afternoon."
Like I said, Ni draws out bigger plans. Si wants exactitude. That's why their plans are more specific. There's more to elaborate on but my eyes are closing. I'll post more tomorrow. It's really about telling the difference between how abstract vs. concrete you are.