How do you guys work with partnerships where you verify the referrals?

How do you guys work with partnerships where you verify the referrals?

For example, I sell products for runners online and I have a special offer where if they spend a certain amount these runners can also get other perks such as a 20% off running classes with a coach and a free trial. So I wanted to create a system where when my client goes to a certain partner they verify that they are my customer. The only thing I could think of was asking the coach to ask for the customer's order number and they need to then check with me to verify. Or do I send that customer a code? Is there a way to automate this? I use shopify so if anyone knows an appropriate app let me know.