Please keep in mind posting a form directly from a website directly to Marketo is not allowed. It will raise an error.
All form submissions must go through Munchkin JavaScript API or SOAP API.
Once the data is submitted, no matter the method, you can retrieve all fields and respective values using SOAP API's getLead or getMultipleLeads.
The lead attributes will display each field and respective value. A real example may help to clarify:
<leadRecord>
<Id>1070246</Id>
<Email>test@marketo.com</Email>
<ForeignSysPersonId xsi:nil="true"/>
<ForeignSysType xsi:nil="true"/>
<leadAttributeList>
<attribute>
<attrName>AnonymousIP</attrName>
<attrType>string</attrType>
<attrValue>193.95.0.0</attrValue>
</attribute>
<attribute>
<attrName>Company</attrName>
<attrType>string</attrType>
<attrValue>marketo support test, please ignore</attrValue>
</attribute>
<attribute>
<attrName>FirstName</attrName>
<attrType>string</attrType>
<attrValue>marketo support test, please ignore</attrValue>
</attribute>
<attribute>
<attrName>InferredCompany</attrName>
<attrType>string</attrType>
<attrValue>Esat Telecommunications Limited</attrValue>
</attribute>
<attribute>
<attrName>InferredCountry</attrName>
<attrType>string</attrType>
<attrValue>Ireland</attrValue>
</attribute>
<attribute>
<attrName>Job_Posting_Status__c</attrName>
<attrType>string</attrType>
<attrValue>No Job In System</attrValue>
</attribute>
<attribute>
<attrName>LastName</attrName>
<attrType>string</attrType>
<attrValue>marketo support test, please ignore</attrValue>
</attribute>
<attribute>
<attrName>LeadStatus</attrName>
<attrType>string</attrType>
<attrValue>Open</attrValue>
</attribute>
<attribute>
<attrName>Mailing_List_Hiring_Tips__c</attrName>
<attrType>boolean</attrType>
<attrValue>1</attrValue>
</attribute>
</leadAttributeList>
</leadRecord>