Get a piece of glass and crack it with your camera photograph under the lighting conditions of illustration, place it in your illustrations document and trace it using the pen tool add adjustements and effects as required.
Otherwise look for tutorial on Googles for Crack Glass Tutorial for Adobe Illustrator.
Get a piece of glass and crack it with your camera...
1/ S/He's looking for a mirror, not plain glass – despite the topic head
2/ Bad luck to break a mirror...
3/ Crack it with her/his camera???
Just kidding Wade, couldn't resist it!
Funny, I did a job like this for last Friday 13th.
The first thing was to draw the cracks:
This was done with the pen tool, traced over a photo.
The final stuff was done on a plotter using self-adhesive mirror foil.
It looked something like this but completely fooled the eye when stuck onto a real mirror: