The troubleshooting step being used by the helpdesk technician when leading the caller through checking power cables and power lights is "Identify the problem." At this stage, the technician is trying to clarify and confirm the exact issue the caller is experiencing by guiding them through basic checks to understand what is meant by "the computer will not turn on." This step is essential before establishing a probable cause or implementing a solution. Once the problem is clearly identified, the technician can proceed to the next steps of troubleshooting effectively.