Practicing with slicers

Just realized that apparently you can use/try slic3r, makerware, cura, s3d etc without having a machine, so that you can get the feel for it first. Is this true?

Also, although makerware covers up most of the code, can you still get inside to make fine adjustments like with the others?

You can use the slicing software you listed to create gcode files, which are instructions to the specific printer you're using. You don't need a printer to create the gcode. Google gcode viewer and you'll be able to find software you can view the resulting gcode with.