Without JS, this will be not that easy Still, you can use hidden fields that capture URL parameters.
For instance, when a lead enters a page on the site with a Marketo embedded form, you will need to have the URL to look like:
http://www.yourdomain.com/yourpage.html?content=[Name or ID of the content]
Then you will be able to capture [Name or ID of the content] using a hidden field from the RL parameter (see Set a Hidden Form Field Value - Marketo Docs - Product Docs ).
Finally, in Marketo, you will be able to use this field value as a filter together with the "field out form" trigger to know what content is requested.
The Referrer of an embedded form will contain the full URL of the hosting page (the page on your 3rd-party site). You can create triggers based on string matches. Just understand that Marketo does not have an actual query string parser, so make your params as distinct as possible. (Don't use utm_campaign=barf and utm_campaign=barfly because they will both match Referrer Contains "utm_campaign=barf".)