<p>When using Slic3r I noticed that <code>Slice now</code> and <code>Export G-Code</code> do different things. While <code>Slice now</code> is nice, it does not show any tool paths etc. </p>

<p>Is there an actual way to generate and visualize the G-code in Slic3r without saving the exported G-Code first? When aligning seams etc., it is quite annoying to always save the file to see a difference because <code>Slice now</code> seems to make little difference.</p>

After you use the Slice Now button (and the slicing progress bar shows completed), select the preview tab. To the right of the window you will see a pair of vertical sliders. Each slider changes the start and finish locations for the filament layers.
You can slide the left one to the bottom, which will "empty" the virtual print bed. Each movement upward will display a succeeding layer, showing the placement of the filament. This ostensibly will present to you alignment information as well as unexpected holes or other failure points.
I don't know of any slicer that lacks a preview. That doesn't mean one does not exist, but why would such a useful feature be omitted?