I just use a strong flashlight (mine is a Maglite). I also have a homemade candler that I made, but I like the flashlight better. Some candle more than others. You can start around day 10. Brown eggs can be a little harder to see but it's possible.
thanks for the pic, i'm on day 7 of duck eggs and can only see a dark spot in all of them(i assume its the yolk). am i doing something wrong or do they need more time. its my first batch and i'd hate to have a 0% hatch rate. i'm hoping i dont see any veins yet because the ducks shells are thicker.