How do I setup a go/not LED switch?
In the circuit shown below, when the DUT's resistance is less than 1000 ohms, LED1 will shine with constant brightness but when the DUT's resistance is greater than 1000 ohms the LED will go out completely.

When the DUT is equal to zero ohms there'll be 500 microamperes through it, and at 1000 ohms the current will fall to 450 microamperes.

U1 is a voltage regulator, and will drop the battery voltage down to a steady 5 volts for the comparator (U1) and is used - along with R1 and R2 - to generate the comparator's reference voltage.

I've shown an LT1716 for the comparator, but any comparator capable of sinking the LED current (about 10mA for the LED shown) will work. 

To set up/calibrate the tester you'd use a 1000 ohm known good resistor for R4 and adjust R3 (a 10 turn trimpot would be nice) until the LED just went ON or OFF. Thereafter, any resistor with a resistance greater than your reference resistor would cause the LED to go OFF and with a resistance less than your reference would cause the LED to go ON.