How do I retrieve the custom field ID of a custom field for my web2leads form?

If you are logged in as an Administrator and visit the following link, it will give you a JSON output of all the custom fields (deal, person, and company) and all the dropdown selections available: https://api.pipelinecrm.com/api/v3/admin/custom_field_labels.json. 

People specific Custom fields, you can use this link: https://api.pipelinecrm.com/api/v3/admin/person_custom_field_labels.json