Recognizing the CDV is simple, look for a photograph on very thin paper, usually an albumen print, but later occasionally seen in gelatin-silver or rarely even platinum.The main feature, as mentioned above, is the card size, which is typically 2.5 x 4 inches (63 x 100 mm) or slightly less, though you need to notice if it has been cut-down by the owner to make it easier to fit in a photo-album sleeve.
But CDVs and Cabinet cards had a different look to them -- they have prints that almost fill the whole card surface, with very little margins.
Often the bottom margin (or sometimes the side, if the image is in landscape orientation), has a wider margin to allow a caption or photographers imprint, but the other margins are very thin.
They still made CDVs in the 1880s and 1890s, but not in any great numbers.
The cabinet card is said to have been introduced in 1866, but it was used almost exclusively for landscape images in the 1860s -- CDVs were just too small for a nice view of the Rocky Mountains or Yosemite.
That frame has two tabs that extend backward, and pass through slits of the correct size in the CDV sized cardboard, and when folded back hold the tintype in place.
I knew some of these date from the early 1860s from the thickness of the card mount used, and eventually found one dated 1862, but do not know if they were used in the late 1850s, or for how long they were produced.
Hence I developed my Card Mount Criteria to distinguish between true card mounts, and simple matted images. ): To be a true card mount, the print should be 90% or more of the total card width or height.
This criteria does not apply to stereo-cards, though they often meet it anyhow.
I realize I may be swimming upstream on this minor distinction, but it becomes relevant with cabinet cards and post-1900 matted prints on cabinet card sized mounts.
The last true CDV in our dated images collection is from 1899. They typically measure 4.25 x 6.5 inches (108 x 164 mm).
Here in the USA, photographers were just getting used to adding ambrotypes to their offerings.