Early preservers were fairly plain, usually adorned with simple patterns.
The preserver was prone to breaking at the corners.
These cases are not rare, but they did not dominate the daguerreotype market, probably due to price.
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.
Note: These were used with daguerreotypes, ambrotypes, and tintypes General Rule: Thermoplastic or Union case dates after 1856.
Once you have determined that your image is a 'dag', several clues can help establish a date range.
Most clues are evident by just looking at the image. This is not recommended for the amateur or skittish.
The preserver is a brass frame that wraps the glass, mat, and image.
Early daguerreotypes were protected by only a mat and glass, and then placed in a case.
They changed significantly about three different times, with lesser evolutions along the way. The earliest ones had a 'pebble-like' texture, and were usually octagon or oval up to about 1845.
About this time the finish was a little nicer, having a 'sandy' texture. These included the nonpareil, acorn, elliptical, double elliptical, and some other variations.
The daguerreotype is very easily detected, even by someone who has never seen one before.