Fill out the form with your own info and you will see the form fill (and Form Name) in Activity History.
Also, you can alternatively view the page source of the form page.
Find the string "mkto_####" as such
From there, go to Marketo and in the URL after "marketo.com/" type #FO[insert the number here]
So, my example would be https://app-AAA.marketo.com/#FO785
That will bring up your form.