Output a data file from any program that has data that may be incorrect within a field. Load the file into a database program and perform a grouping query on a field in the table. Count the results per value. Look at the results. With luck, correct values will grossly outnumber incorrect values.
Variations are easy to spot.
Remember that data quality preservation is a constant activity and periodic repairs are used to support a regular maintenance program, not replace it. Once repairs have been made, set up regular maintenance checks. Review the reports. Define data entry edit checks can be built to prevent incorrect values. Note that data entry checks of a certain form may not be feasible.