A part with no label
Count the legs, look at the body, and then let the meter break the tie. Ohms range separates a resistor from a potentiometer, a thermistor and a light sensor. Diode range separates a diode from an LED. Six touches separate a transistor from a MOSFET.
Start with the legs
Two legs, three or four: that cuts the list at once. Then the body. Answer in turn, and the figure ends on a name and the test that confirms it.
The meter turns every guess into a reading. How to set it up is in Your multimeter; here it is only a question asked of the part. Always measure a part on its own, out of the circuit and with the power off.
Ohms range separates four parts
A resistor, a potentiometer, a thermistor and a light sensor all read in ohms. What separates them is what happens when you disturb them. A resistor reads its colour code and does nothing. A potentiometer has three legs and a middle one that slides. A thermistor falls as it warms, and a light sensor falls as it brightens.
Diode range separates a diode from an LED
In diode range the meter pushes about a milliamp through the part and shows the voltage it took. With the red probe on the anode that is the forward voltage, and with the probes swapped the part blocks and the meter shows OL.
The numbers are typical. Your 1N5819 may read 0.15 or 0.3 V and your red LED 1.7 or 1.9 V, and the colour is as good a guide as the reading is.
Six touches separate a transistor from a MOSFET
Both are the same black half-round with three legs. Read the print on the flat face first. If it is gone, the meter settles it.
A transistor reads a diode drop from the middle leg to each of the other two. A MOSFET reads one diode between two of the legs, with the third open to everything. The meter finds the base, or the gate, with certainty. It cannot reliably say which of the other two legs is which in the transistor, and for that the maker's sheet is the authority.
When it does not work
That is the meter, not the LED. Diode range sends about 3 V at most on many meters, and a blue or white LED needs about 2.7 to 3.1 V before it conducts. Try a red LED to check the meter and the probes, and read a blue one by building the circuit in the first experiment.
Take it out of the circuit and keep your fingers off both probes at once. A resistor still joined to other parts reads the resistance of the whole path, and a finger across the wire reads about a megohm in parallel with it, which matters on the large values.
That is probably a MOSFET with a charged gate. The gate is a tiny capacitor, and a static charge on it can switch the channel on. Touch the gate to the source with a wire, then measure again.
A jump of a few percent is dirt on the track, and it settles after the knob has been turned back and forth a few times. A jump to open or to zero in the middle of the range is a worn track, and the part is finished.
A bead that small follows the air around it, and your fingers are warmer than the room. Pinch it between two fingers and wait ten seconds: the reading should creep downward and keep going. If it does not move at all, it may be a resistor with the print rubbed off, not a thermistor.
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