The main thing about a file printabibilty is wether or not it is manifold in other other words it has to be a closed shape with no holes.
Netfabb is very good at detecting these and repairing them, once you have opened your file check for any red parts in the model which indicate a fault. If so then press the red “+” button at the top and follow the automatic repair procedure. Before exporting the repaired part: Part -> Export -> Stl.
Another problem but less common is a hight poly count (high definition files) which can make the slicing tricky if not impossible. Again it’s an easy fix with MeshLab: Filters -> Remeshing, simplification and construction -> Quadratic Edge Collapse Detection. Although this might sound complicated you’re basicly reducing the files size whilst trying to keep all the details which you will be able translate into the print. For reference this process is called “Mesh Decimation”.
When starting long prints I also find it useful to check the gcode using this website: http://gcode.ws/
If anyone has any good tutorials on mesh handling please share
Edit: I forgot to mention the thin wall issue, as an example when using Slic3r any walls smaller then the nozzle itself won’t be detected and therefore printed. Again walls can be measured in NetFabb when clicking on the ruler, top right of the screen. Be sure to select to correct measuring tool at the bottom right.