You can find leads that have responded to any automated correspondence in the workflow by clicking on the Conversations tab in the sidebar menu. Here are 2 ways to respond to leads.
Email Responses
To reply via email, locate the email thread that the contact was responded to. This will indicated by a number in parenthesis to the left of the subject line. Click the 3 vertical dots to the right of the email. Select Reply in the popup box. This will sent a response to the contact within the email thread.
Do not reply to emails using the message box at the bottom of the middle Conversations panel. This will send a separate new email to the contact which may cause confusion. Always reply within the email thread.
SMS Responses
To reply via SMS, use the message box at the bottom of the middle Conversations panel. Be sure to click on the SMS tab to submit a proper response.