Fortunately you can identify a 'real' hallmark jeweller even without walking into the jeweller's store. If you look at that Hallmarking process: In order to sport the Hallmarking Store (accompanied with the blue triangle) A jeweller has to get the Hallmarking license through BIS. Once approved you can start advertising yourself as a Hallmark jeweller( of course you need to keep hallmarked jewellery too). This way the governing body BIS maintains a database of all the approved jewellers throughout india.
That database is available in the public domain at www.bis.org.in. On the homepage look for 'Locate Hallmark Jewelers in your city' link and the rest is fairly easy. Once you select your appropriate state it will spew a PDF of all the active licensees. This way you can verify that the jeweller is a True hallmark jeweller or not.
The tough part is the link may not always work. I have written to the IT dept of BIS but to no avail. I have a copy of all the indian hallmark jewellers. We go by Kathana Jewels Pvt. Ltd. and the address is the same.
You can view/download the file here (thank you Google docs!!!).