Ensuring the accuracy of data entered is the most important function of a business accounting system. Many accounting and software programs lack correlation edits, which help weed out data that does not make sense. Without these edits, incorrect data can easily be entered and missed. Look into scrubber programs that run at night and alert you to suspicious entries. Using passwords, installing firewalls and monitoring workstations are key steps.
Fraud can still occur when someone signs on using another person's password.
Make sure your system has an automatic lockout feature. After failing to enter the correct password three times, the person has to call the boss to gain access. Require passwords to be comprised of both letters and numbers, no names or words from the dictionary. Also, require employees change passwords periodically. Most systems have audit functions that track who is responsible for specific data.