Comparing the addresses of photographers printed on cartes-de-visite or cabinet prints with the dates they appear in directories should give an approximate date of the photograph.
I have used directories held at the Guildhall Library, London, the Society of Genealogists and the British Library.
the business known as Thomas Forrest was at Market St, Pontypridd from 1868 to 1926.
In such cases, if you have several cartes from the same photographer, a comparison of styles of mount could help.
What the sitters wore can also help date a photograph.
The books by Avril Lansdell and Robert Pols are useful for this.
Broadly speaking the younger the adult the more fashionable their dress and hairstyle are likely to be.