I review employee accounts I usually use the system as Jokerman stated instead of statements, generally it's easier to see patterns on the system history rather than on the statement itself (depending on how your statements are set up). Again like Jokerman said look at the position of the employee, do they have access to cash which could be embezzlement. Also, look for cash kiting, are they writing checks when they don't have the funds to check cashers or stores that will give cash back and depositing the cash back into their account before the check clears. But I agree that it's not reasonable to ask every employee to document the source of cash when depositing.
"The reason I talk to myself is because I'm the only one whose answers I accept."
- George Carlin