Gold, platinum, palladium and silver are rarely used in their purest form but instead they are normally alloyed with lesser metals in order to achieve a desired strength, durability, colour etc. It is not possible to detect by sight or by touch the platinum, gold, palladium or silver content of an item. It is therefore a legal requirement to hallmark all articles consisting of gold, silver or platinum, if they are to be described as such (subject to certain exemptions). After an Assay Office has tested the precious metal content of an article it applies a permanent hallmark (stamp) to the article to confirm the precious metal content.
Any article described as being wholly or partly made of gold, platinum, palladium and silver that isn't specifically exempt.
Articles below a certain weight are exempt from hallmarking. The exemption weight is based on the precious metal content. Except in the case of articles consisting of precious metal and base metal in which case the exemption weight is based on the total metal weight:
More details can be found here.
In the case of (2) simply click on the Assay Assured Jewellery Retailer logo on the retailer’s website and follow the link “report a query on this retailer” on the retailer’s Assay Assured Jewellery Retailer certificate.