Well, if you're leading people to fill out the form with UTMs appended in the URL, you can store the UTM params in the hidden Marketo form fields that'll let you know which SEM-keyword brought in the engagement. There's a way through which can you can essentially store the UTMs in a cookie which can then be used to fill the hidden fields on the form when visitor fills form on your website w/o UTMs appended, you may refer that here. However for a known person visiting the webpages with the Munchkin placed, you'd have the UTM params logged in the visit web-page activity using which you should be able to gauge the performance of the SEM keywords. One thing to note here is that the visit webpage activity in Marketo is only persisted for a period of 90 days from the date of activity.
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