Unfortunately, there are not always visible warning signs before a lithium-ion battery fails. However, some tell-tale signs that the battery may be failing include:
Overheating: if the device containing the Li-ion battery becomes unusually hot to the touch, especially near the battery area, it could be a sign of a problem and you should not continue to charge or power the device/battery.
Swelling or bulging: a failing Li-ion battery will often expand, causing the battery to swell or bulge. If you notice visible deformities to the shape of the device and/or battery, stop using the device immediately.
Reduced battery life/inconsistent charging: if you notice that the battery life of your device has significantly decreased, or the charging process becomes erratic or unreliable, these can be signs of a failing battery.