Usually their CRM. Hubspot has a free CRM to start with, but it's really whatever the client is comfortable using. As long as we stay on top of them about updating and keeping records accurate, then we get a fantastic feedback loop on our marketing efforts.
When actual revenue / deal stage isn't available, then yeah it's a LTV converstation to get an average value of a customer. Knowing how many marketing qualified leads get sent to sales, and how many of those were qualified can usually be pulled out of the CRM the client is using.
If your clients aren't using a CRM of some kind today, then get them on one. It makes their life easier because they can keep track of customers, and makes your life easier by getting real data on your efforts instead of guesses. There will be pushback initially, but once they start seeing the power of it they will most likely dedicate resources towards managing the system.
We just had one client bump hours for a part time employee up because she's now working on follow up for any leads that didn't respond after an initial phone call/meeting. Because of the Hubspot free CRM they saw they had a bunch of leads that were slipping through the cracks. Following up and getting one of those leads back into the sales cycle could mean a $300k-1mil job.