You can preview a part in the queue at any time by clicking on the part's icon - an automatically generated silhouette of the part as it will appear from the top looking down. Clicking the part's icon will open the preview window. A rotatable, zoom-able 3D representation of your part will appear in the 3D View. Certain types of files (such as pre-generated .gcode files) cannot be previewed in this way.
Parts can also be previewed in Layer View - a layer-by-layer representation of how the Me2 will construct your object. The Layer View is dependent on your current print settings and is only generated on demand - to create the layer view with your current settings click the "Generate" button.