Can you please elaborate a little more on your question? I am assuming you are asking if a model can be thought of as a sheet metal component (which can be created using NX Sheet Metal) or you need to use solid modeling for the same.
If you want to analyze a dumb solid body..then check for the thickness (sheet metal components will have majorly a uniform thickness throughout). If you have a large variation in thickness..go for solid modeling tools. (although plastic components may have uniform thickness but you can easily figure it out by the design features like Boss/snap fits/etc.)
Are you asking how do you know if a body is a sheet body or a solid body in the model? The quickest way to do that is to change your selection intent in the drop down menu to "Sheet Body" then move your cursor over the geomentry. If the geometry highlights then you are dealing with sheet bodies. You can also do the opposite, change the selection intent in the drop down menu to "Solid Body" then move your cursor over your geometry. If the geometry highlights then you are dealing with solid bodies.