First, get the last court order which set the amount to pay. Then figure out how many weeks since this court order. Then do the math like this:
A few things to keep an eye out for: 1) for new paternity cases, the judge can actually go as far back as the birth of the child, not necessarily the day of court, and 2) make sure you count from Friday to Friday (No, it does not matter that the order made was on a Monday). Get a calendar to help with counting every Fridays.
Go the county clerk office. The support clerk keeps a record of all child support payments. Tell them you want a copy of your support docket. Using the support docket, subtract the payments from the arrears number. That will give you your arrearage figure.
Remember, this is just a general and generic process that may give you a rough idea. If you are the person receiving child support, you have just saved yourself some money when you go to a lawyer to get your child support collected from the payor. Payors can figure out how much trouble they are in.