Suppose you measured 485V at the supply of a motor and 430V at the motor. Your voltage drop calculations show that voltage at the motor should be 470V if there are no losses in cabling or connections. Congratulations. You've discovered power distribution system maintenance problems that are reducing motor life. You have serious losses in the cabling and/or connections. These problems probably exist well beyond that one feeder.
You can use thermography to find high-loss connections. But if you just go in and repair those, you may end up doing that work twice. Yes, identify those connections; they will have to be replaced. Before you replace them, perform insulation resistance testing on the feeder cabling.
After you replace defective cabling and connections, repeat the insulation resistance testing to get baseline readings. Why is this important? Because those readings will allow you to properly trend readings when you do the "lessons learned" thing and make cable testing part of your predictive maintenance program. Thermography, too, will be part of any sound program.