How do you Quantify Upsell sourced by the CS team?

Hi Nicole, our team is responsible for executing on transactions, but one process we have that sounds parallel to yours is Referral Qualified Leads. These are leads in SFDC that are created when one of our clients refers us to someone in their network, and the BD team then divvies out the lead through their usual qualifying process to determine if it's a Sales Qualified Lead.
The RQL, in our setup, is like a Marketing Qualified Lead in that we are adding to the top of the funnel.
You could start tracking "CS Qualified Leads" in the same way. Google Sheets is always great to get things off the ground and then you can learn what fields (or object) you need to build in SFDC to replicate the tracking and desired metrics there.

We also use Lead Source field on the opportunity to track upsell referrals or leads from our CSM team.

Adding to Denise's comment, at Gainsight we've added a custom field to the Opportunity specifically for Upsell Lead Source. This is a required field to close/won the Opportunity and contains options like:

Inbound Request
Outbound Effort (by AE)
Outbound Effort (by CSM)
Outbound Effort (by Exec)