RFID is a promising technology for anti-counterfeiting since it facilitates processing of product information. In large scale RFID applications (such as - supply chain, retail industry, pharmaceutical industry, etc.) tag authentication is used to detect counterfeit products. However, RFID authentication protocols are mainly per-tag based where reader needs to authenticate tags sequentially one at a time. This increases the protocol execution time due to large volume of authentication data. In this paper, we propose to detect counterfeit tags in large scale system using efficient batch authentication. We propose FSA based protocol, FTest, to meet the requirements of prompt and reliable batch authentication in large scale RFID applications. FTest can determine the validity of a batch of tags with minimal execution time which is a major goal of large scale RFID systems. FTest can reduce protocol execution time by ensuring that the percentage of potential counterfeit products is under the user-defined threshold. The experimental result demonstrates that FTest performs significantly better than the existing counterfeit detection approaches, e.g. authentication techniques.