Take the contacts marked marketing qualified in one quarter and follow that same group forward. Count how many of them sales later qualified and how many ended up with a deal. Staying with one group matters, because a deal that closes this month can belong to a lead created months ago.
Suppose 40 contacts reached marketing qualified in the first quarter and sales qualified 10 of them. The reps report that most of the rejected leads held purchasing titles and had no say in software decisions. The company could limit the title points to operations roles and then watch the next quarter's group.
HubSpot's attribution reports add the source side. Contact create, deal create and revenue attribution are separate reports, and the deal and revenue reports require Marketing Hub Enterprise. Each model allocates credit to interactions HubSpot recorded, such as tracked page views, submitted forms and logged calls. The deal create and revenue reports show which pages and campaigns appear on the path to a deal. These reports don't prove that a page caused the deal, and they can't count an interaction that was never recorded.
Use what the group shows to revisit the criteria in your definition and your choices about where leads come from. A rule that keeps passing leads sales rejects is worth a second look, and so is a source whose leads sales keeps accepting. A low acceptance rate can also come from how reps qualify, even when the criteria are sound. And a source whose leads sales accepts still has to be worth what those leads cost to acquire.