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 – Schematic created using CircuitLab
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.
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.
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