From my observations, both are similar in terms of sociability, rationality & emotionality. For instance, you would not be able to tell the difference if they were keeping to themselves with some activity; or were placed in some social situation that they obviously would not like to be in or do not agree with. This is because they are temperamentally alike and appreciate overlapping interests, skills, and occupations. Their similarity is marked by annoyance to intrusions, avoidance of displays of emotion, and antisocial & cynical bents(although not every IxTP is extreme).
Where they do differ, in my understanding, is the preference for information gathered directly from reality(Sensing) versus information gathered indirectly(Intuiting). The best way to explicate the difference is to say that: (1) INTPs are Observers in their environment and ISTPs are Experimenters or (2) INTPs are Intuitive Engineers and ISTPs are Sensory Engineers.
Where ISTPs tend to be craftsmen, mechanics, and builders, INTPs do the same, conceptually, with abstract thinking. INTPs have a noted capability with problem solving, engineering the world around them to fit with their ideas and make the (previously) unoperational work. As for experience, as I said ISTPs are experimenters and outshine INTPs when it comes to real life hands-on practical knowledge, implementation and exposure. You will find that ISTPs have done things most INTPs are too lazy or anxious to do. INTPs in turn tend to be more hands off observers, theorizing about things in their abstract state, which usually means they tend to have more of a penetrating understanding of the workings of an object or subject. INTPs also are more inclined to accumulate knowledge than the experiential ISTP, even seeming 'smarter' or more intelligent.
So, if you really want to tell them apart, observe how they handle day-to-day life, and how they cultivate and work with their experiences in life.