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 folder under DigiUSB.

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!