In Segment Step
The In segment step checks the current lead's state at execution time. It is available only in For dialogs automations.
Select A Segment
Add the step and open its settings on the workflow. In Segment, select a segment in the current project.

The step uses Yes when the lead is currently a member of that segment and No when the lead is not. This checks current state, not the event of joining. To start a new run when a lead joins, use Added to segment.
Connect Both Branches
Connect the Yes and No outputs. If one branch is left unconnected, the editor shows a warning and a run that chooses it ends at this step. Execution requires a current lead. If the selected segment is deleted or unavailable to the project, the workflow does not pass validation before publication.