Check out the Digispark O-Scope example
Load the Examples→DigiUSB→DigiScope sketch on to your Digispark.
Connect a probe/wire to ground and to Pin 2 - use like an oscilloscope!
Run the Digiscope binary (windows) or the digiscope.py python script in the Example Programs here: https://github.com/digistump/DigisparkExamplePrograms
It is calibrated for the approximate voltage when plugged into USB (4.75v once it gets to the Digispark through the diode) - so it should measure 0-4.75v
It samples as fast as it can given your computer and how inefficient the graph rendering is - not a highly detailed output or anything but a great quick and dirty tool for troubleshooting!