While use of a data discovery tool is not required by PCI DSS, they may be able to assist with the scoping process. There are many different tools available that can be used for cardholder data discovery. The tool or tools you use is very dependent on a number of factors including budget, environment, and resources available, to name a few.
There are commercial products and open source solutions available. Your organization may already have a tool that could be used for cardholder data discovery either by itself or in conjunction with another tool. Many organizations have data loss prevention products installed and while they may not be currently configured to detect cardholder data, with some minor adjustments they may provide a viable solution.