analog Voltage Reader

I'm sure this is a really simple answer but I'm completely forgetting it as I haven't worked on my Arduino in a month. What would the code be if I wanted one of my analog inputs read a voltage coming in. Not from a sensor or anything, just a voltage. It's going to be the voltage from my power supply so my robot will alert me when it's running out of battery. Any help would be great! Thanks! :)

This has been discussed a number of times, here is one of the threads:

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