Recently we have noticed many low cost "Toshiba" 2SK170BL and 2SJ74BL in the market.
To identify whether they are genuine, the serious method is by putting them on curve tracer.
Otherwise, a reliable way is to use ohmmeter to measure the resistance between drain and source. Genuine 2SK170/J74 has a resistance of 50 ohms or below. Fake ones have resistance over 100 ohms. It was heard that the fake ones were remarked from 2SK246 /J103. But when these FETs are also out of production, people remark 2SK30ATM for 2SK170BL. 2SK30ATM has much lower admittance( thus gain) than original 2SK170 so the fake ones cannot function well in a 2SK170 circuit.