Most clues are evident by just looking at the image. This is not recommended for the amateur or skittish.
The image, case, or brass holder can be damaged easily.
Once you have determined that your image is a 'dag', several clues can help establish a date range.
General Rule: An image with a preserver dates after 1847.
Early cases were usually leather-covered wood cases. These were basically the only type of cases until the latter part of the 1850s.
A thermoplastic case is recognized by its smooth plastic-like surface, one-piece construction (of each half) and brass hinges connecting the two halves.
They are frequently broke or chipped as cases were brittle.