A potential divider which converts 15V to 5V fed into a PICAXE ADC input, an accurate 5V supply rail, a means of display, and a few line sof PICAXE code. So that's two resistors, a PICAXE and an AXE033 serial LCD module for the most basic PICAXE solution.
For more accurate readings you'll need an accurate or adjustable potential divider and/or an accurate power supply or a reference voltage to allow PICAXE software to compensate for any inaccuracies and fluctuations.
Depending upon the voltage source you may need to buffer the input volatge through an op-amp and that can add complexity if you need to go down to 0V or up to 15V.
You could of course just buy a digital panel meter which would probably only need a power supply plus single resistor to configure and calibrate.