How do I know if additional withholdings employees have allocated for Federal taxes are coming out monthly or per pay period?
You can review paystubs and compare them to tables in Circular E (Pub 15) from the IRS. But all FWT including any extra should all be coming out per pay period. If an employee wants an extra $520 taken out and you pay weekly that would be an extra $10 per paycheck. It will not be listed separate on the paystub and there is no way internally to know