You can tell if a photo is a daguerreotype in two ways. Daguerreotypes will reflect in a mirror like way, making the image look kind of like a photo negative when it is held at an angle. Ambrotype images stay consistent at an angle and if you VERY carefully remove them from the case and remove the gold colored foil frame you can peek at the reverse side. If it is painted black, you have an ambrotype for sure.
If it is an ambrotype it dates to a pretty narrow window of time. This technology was discovered in 1854, was at it's peak of popularity 1856 -1860 and fell out of favor 1861 - 1866 when the more durable tintype images grew in popularity. This site has a lot of good info and helped me identify my ambrotype:
http://www.phototree.com/identify.htmDo not under any circumstance remove the clear glass plate that protects the photographic glass plate. If you touch the original image, you will have permanent fingerprints on it. Be very careful with the foil frame too.
People had their photos taken for all types of occasions (like marriages). They were not limited to death or because death was imminent. Photography was all the rage and everyone wanted their picture taken!