A simple test for small signal level JFETs from the junk box

Connect up the JFET to be tested in the circuit above. Set the variable power supply to around 9V. For a small signal device, say a 2N3822, you should see around 3mA of current. This current will be a little lower for lower power supply voltages, and a little higher for higher voltages. Like the purple curve in the graph. This purple curve will not exactly follow the black curves, as VGS changes a little as the current also changes a little. You'll get a rough idea of where the JFET is most linear vs supply voltage. If you see this above behaviour the JFET under test is likely good. The PN junction inside the JFET would be reverse biased, so you won't see diode drop action if you used a voltmeter between the source and gate.

You can also do a diode test, with a suitable DVM with diode mode, on the JFET to find the PN junction between the gate and either the source or drain before connecting it up in this test circuit. For an N channel JFET you should see a diode junction with its cathode on the source or drain, and anode on the gate. The reverse is true for P channel JFETs. The diode junction measured is the body diode. In the above diagram you can see the PN junctions of an N channel JFET compared to a diode.

What an N channel JFET would look like with a DVM.
Between the source and the drain you should see, with the gate at 0V, a resistance something like 20Ω to 1K, both ways with the DVM.

In most circuits, this JFET PN junction will not see diode action, as it is usually reversed biased. The schematic symbols of both show the anode arrowhead, for the P material in these devices.