For jewellery which you already own and is not hallmark certified for some reason you can get it checked for purity by getting a 'touch report' from the local jeweller in your city who is authorized to do so.
As a cautionary note always purchase jewellery which has a hallmark certification. Hallmarked certified jewellery carries a embossed BIS logo, a three digit number indicating the purity of gold (958, 916, 875, 833,792,750,708,667, 585, 375 - BIS 958 indicates 23 karat gold, while BIS 916 translated to 22 karat gold and so on), logo of the assaying/certification center, code denoting year of hallmarking and logo or code of the jeweller. While purchasing jewellery make sure that you get proper payment receipts from the gold jeweller for the payment you have made against it.
Note: KDM jewellery is not Hallmarked jewellery.
BIS (Bureau of Indian Standard) hallmarking is a system used in the Indian market for certifying the purity of gold serving as a third party assurance between a gold (jewellery, coin or bars) seller and a buyer.