I'm not familiar with US Payroll, but you should start with learning of what information is available in the Legacy System and in SAP¨.
Regarding the information available to the Payroll Department, the Payroll Journal may be a good starting point since it should contain the employee's earnings and deductions plus the employer's contributions. Next will be the Third Party Remittance (or Reports to Finance or Third Parties on the employee deductions and employer contributions) and the FI/CO Transfer (or Reports to Finance on Payroll Expenses).
Regarding the information available to the individual employees, the Remuneration Statement should be the initial document to compare, followed by all other Payroll Reports the employee receives (like the W-2 Forms) or has access to (such as through ess).