How to use MQ-2 module?
As Peter says in a comment, there's a relay on that board. The output connections include COM, NO and NC connections that are usable as follows ...







simulate this circuit &ndash; Schematic created using CircuitLab

1

So set your multimeter to resistance (ohms) and connect it from COM to NC. When there is enough gas to trigger the relay the resistance will increase from 0 to infinity (or at least to "out of range" - some multimeters show that as -1)

You could use the COM and NO connections to switch on a large and loud siren and/or strobe-light.

2

The board also contains an indicator LED. You can connect a voltmeter across the resistor next to it. When the LED is turned on the voltage across the resistor will rise from 0 to perhaps a few volts.

3

The board has an LM358 op-amp, I guess this is processing the MQ-2 output signal, it might be interesting to put a voltmeter on it's output. See LM358 data sheet for pinout