Load testing each cell dynamically would probably give best .
Measuring Voltage after recharge and after use will give an indication of which is bad.
If it's possible to use it with the cover off...there are cell phone simulators which might identify the bad cell/s.
https://prepperswill.com/do-thermal-imaging-app-work/
https://www.bing.com/videos/search?q=Kobalt+40+V+2.5+Ah+Lithium+Ion+battery+ce lls&docid=608022482793334397&mid=996765A2391174D384A5996765A2391174D384A5&view=detail&FORM=VIRE
There might not be individual new cells available, but the tested-good ones could be kept for future problems.