How do I get an Arduino to monitor its own voltage levels?
Use a voltage divider to bring the VCC voltage below 1V, connect it to an analog input, and use the internal 1.1V reference of the ATmega328P:
Alternative, you can measure the internal reference voltage which should be 1.1V, then work out what value the Vref is from the value you read the 1.1V from.
The sensor I'm using is powered by the Arduino's 5V supply. What's the reason for dropping the voltage and using a 1.1V reference instead of the 5V default?
but I don't understand what you mean when you say to "work out what value the Vref is from the value you read the 1.1V from."
The point is that once you change to the internal referance only voltages below that will be measured, so you have to use a potential divider to do that. Say you have a five to one divider, then exactly five volts on the power will give you one volt you can measure. As the five volt supply changes then so will your reading, because the referance is fixed.
Does this mean I can count on the internal reference voltage not to change even if my Arduino's 5V rail were indeed unstable?
but I don't know whether the dips and spikes I'm getting are too brief to register on the multimeter.
The only thing in common between an Arduino and Netduino are the letters "duino". So I'm not sure how much of the article is relevant.
I'm getting occasional dips / spikes in the voltage output of a humidity sensor...