I just wanted to check the battery health of my old Thinkpad in Linux (currently Ubuntu 24.04 LTS), and it turned out to pretty easy. This post here explains it just perfectly: https://www.cyberciti.biz/faq/check-see-laptop-battery-health-in-linux-command/
I tried the the variant using the upower command, and it worked just fine. This is what my result looks like, so roughly 65% of the original capacity:
Just ordered a used one with 95-99%. Will check the same way after I have built it in.