By my experiences, When shown 100%, that doesn't mean a lot. It's a rough guess. It could be 100%, or 80% as well. When It show less than 100, like 90 or less, it's likely in bad condition most of the time. The best way to measure any battery in electronics like phones/tablets/notebooks without actually open any cover is to measure device while charging from zero % to full with USB power meter connected between wall charger and device. When fully charged, read the Wh value, for example 11.5Wh, then do 11.5x0.88=10.12Wh and you'll have pretty precise guess usually +/- 3% how much energy could battery store. Then compare value with what you can find on the internet. When you can find that battery should have 12Wh, then 10.12/12x100=84.3% is the real capacity.
Tips for as precise measurement as possible:
*Connect USB power meter directly into device. Not into charger to avoid cable power losses.
*Don't use the device while charging. Ideally turned off is the best, but turned on on airplane mode with display off is also fine.
*Don't use cheapest power meters because those displays only Ah, or mAh and you can't compare mAh with mAh labeled on battery. That's more complicated to calculate. What you'll need is value displayed in Wh.
When You can't find battery value on internet in Wh, then calculate it from battery nominal voltage and mAh. When Battery has 3.82V and 5000mAh, then 3.82*5.000Ah=19.1Wh