How do i find out quickly when 1 of my affiliates signed up under me?

How do i find out quickly when 1 of my affiliates signed up under me?

So one of my affiliates made a purchase today, but I haven't seen this member before and was wondering when they joined SEO Clerks under me. I thought he/she must be a new member as I didn't recognise the username, but no, my affiliate member count is the same as when I last checked it and also looked at the end of my list of affiliate and not there, the thing is that how do I now find out when that member signed up with out having to going though all pages and pages of my affiliate members to find the username? Is that the only way to do it? or Is there a quicker, easier way to find out?