A few months ago I wrote a post (http://bencbond.com/2016/12/3d-printed-viper-remote-start-key-fob-holder/) on my site that detailed my design process when dealing with complex curves and shapes for 3d printing. To sum up the post I explained how I used the camera on my smartphone and Google Sketchup to make a holder for a remote start key fob. This allow me to do design parts to fit other together perfectly without doing a lot of extra measuring or test prints.
What tools do you use when designing 3D parts with?