A check mark or some type of visual on the series details page that shows it as a series you have completed before (even if you don’t currently have all the cards). Or anywhere that it can be easily seen next to the series title without having to click through to the milestones page.
For me, this would come in handy since there are series’ that I collect just to get the milestone points, but once I finish completing I start trading away cards. It gets confusing when I see I’m at a certain percentage of completion and I wonder “Now did I complete this already and start trading away cards, or am I still in the process of completing this and need to keep collecting?” And it’s a bit of a hassle having to go to view the milestones page each time. >~<