When I design something I use computer aided design. The software presents me with 4 different windows on my computer that I can use to examine what I am making. All the measurements are in millimeters which means that 25 millimeters are about an inch. The top, bottom and side stay in the same angle while the perspective allows me to see the piece by virtually turning it in any direction. The multiple colors in this picture allow me to see different parts of the piece highlighted in color.
When I want to do more detailed work I can expand one of the windows so it takes up the whole screen. This allows me to see up close. In this case I am looking at the back of the butterfly.
After much of the work is done we can create photo realistic renders that will allow me to see what the final piece might look like.
Remembering that these pieces are not as big as the screen makes them can be difficult at times. I keep a ruler in front of me so I can remember.