Trust me, you won't be able to see hairline cracks with 10x magnification. The only way I can test is pushing the battery connector slightly by a screw driver, one side against the shield next to battery connector while gently push it to the left; if it's moving slightly I know it needs to be resoldered.
Any way, this battery connector needs to be resoldered as sooner or later, it will crack. I see someone had a new mainboard installed when sending for repair, but few months later the tablet had to send back with the same problem of cracked battery connector.
Even close up like this, you don't see any cracks
The magic many people fix the problem by disconnect and reconnect battery but most don't understand why.
Look at these pictures you can see why:
This one, pins sit in the middle of the cable tip, this case there is no contact. This tablet will have the same problem even though the connector already be resoldered.
This one, there is good contact between cables and pins of battery connector. This case may be why your tablet is working again.
This one is an example how a repair center fix the problem: solder cables directly to battery connector and re solder this connector