The REST Leads API, of course -- read developer.marketo.com.
Though if you're truly only interested in synced SFDC fields it's better to use the Salesforce API directly.
Also, you should explain exactly what you mean by "for automation purpose," since that could mean anything at all... you're using a Marketing Automation tool after all.
Thanks for quick reply!
I just want to confirm that SFDC fields are added after submitting the form which is on an external website. Do you know which salesforce API expose similar kind of data?
Automation purpose meaning that I am trying to automate the manual test. Hopefully, it's clear now.
Are you asking this same question from the other day again? Because the answer is still the same: you can't call an API from the browser (that includes the SFDC API).
And you haven't explained -- neither here nor in the earlier thread -- what you're trying to accomplish. The whole idea of a successful form post is that it was processed (which, to be exact, means the form data was queued for asynchronous upsert into the Marketo db, it doesn't mean the data was saved completely in the database for the next hit, even if it gets upserted 10ms later).
Please explain the underlying business need that would make you think about this kind of redundant confirmation.