This test won't tell you if the MOSFET is getting weak because the development of hot spots can slowly deteriorate the MOSFET die . This is caused by prolonged stress from over heating . This can reduce the current rating of the MOSFET because the die heats up faster and can no longer pass the rated amount of current. Excess heat can be slow death for a MOSFET .
This test can tell you if the MOSFET has completely failed .
Step 1 Diode check
Set DMM to diode and connect negative to drain and positive to source . Reading for a N-Channel should be around 0.3V to 0.9V . On a P-Channel it should have no reading . Diode is OK
Reverse the DMM test leds . N-channel should have no reading and a P-Channel should read around 0.3V to 0.9V . Diode is OK
0V in either direction means diode is shorted
Step 2 Resistance and continuity check
Set DMM to Ohms Drain to source should be very high resistance with test leds in either direction . Drain to source is OK
Very low to 0
the Drain to source is shorted . MOSFET is defective
Gate to Drain should read very high resistance . In fact you may even see the resistance climb fairly quickly to very high . The is means the gate is charging . Reverse the leds and you may see the gate discharge then charge again quickly . Gate to Drain is OK.
Do the same from gate to source . If that checks the Gate to Source is OK . Don't worry you will not hurt the MOSFET .
If the gate to drain and/or gate to source is shorted or very low resistance the MOSFET is defective.