Identify leads created by Web-to-Lead

To show that a Lead has been created using Web-to-Lead:

• Adding the Lead Source field when creating the Web-to-Lead code
• With the assistance of the web master or web developer, auto-populate the field with the value of the Lead Source by modifying the generated HTML code and set the field to hidden.

Note: Alternatively, Add or Edit Picklist Values to update the Lead Source field with multiple values (e.g. the web page names) depending on the page that the Web-to-Lead code will be embedded into; that way if you have multiple Web-to-Lead pages created then it could identify which website it is coming from.
 
Sample of a code: 
<input id="lead_source" name="lead_source" type="hidden" value="Desired Lead Source Value" />
 

Notes:

• Salesforce offers only the standard Web-to-Lead form and does not support any modifications to it. Additionally, Salesforce does not provide an option to view or edit an existing Web-to-Lead form. The only way to make changes to a Web-to-Lead form is by manually updating the HTML code on your website. As direct editing of existing Web-to-Lead forms is not supported, we recommend creating a new form and using it to update your website as needed.
• Changes are to be decided by the organization's administrator or the webmaster deploying the code to the website.
• If you're facing an issue with records not being created, we encourage you to review the default Web-to-Lead code and see if it works as expected. 
• If so, you'd need to review what modifications have been introduced to the code we provide.
• To confirm the default Web-to-Lead code is working or not, please go to set up | Web-to-lead | Generate web to lead form and save it in html format. You can open the html link to submit details. After it is submitted, lead should be created in org.