How do I check the battery capacity of misc. devices?

Hey iFixit community!

I would like to know if anyone knows of a good way to see specifications for lithium-ion batteries that are not for smartphones or computers?

Specifically I am wondering: How do I know if my DualShock batteries only have e.g. 50% left of the original design capacity?

Are there any out-of-the-box products out there that I can use to test these misc. kinds of lithium-ion batteries, or more exciting: Can I build one myself at home? ;-)