This article may help - it lists the default Marketo fields in the company and lead tabs of a record. You can assume any outside of these are custom or synced from your CRM.
This isn't a very good way because you have to check each field but custom field type can be changed whiel out of the field's type are fixed. Select the field -> Field Actions -> Change Type
Looks like most Marketo fields have "mkto" or "marketo" in the name, but its not consistent. Salesforce also indicates the install source of fields, the date and source can provide some clues.