derek_tooling's FLIR tip is gold — we do the same thing. But I want to talk about the stuff inside the drives that you can't see with a thermal camera.
DC bus capacitors. They're the #1 age-related failure in servo drives and they degrade silently. After 7-8 years the ESR (equivalent series resistance) starts climbing, which means the bus voltage gets ripple under heavy load. You won't see a fault until the ripple gets bad enough to trip the overvoltage or undervoltage protection — and by then the cap is toast and maybe took the IGBTs with it.
How to check: most modern drives have a capacitor life monitor buried in the diagnostics. On Yaskawa it's Un025, on Mitsubishi it's a maintenance counter in the servo amplifier diagnostics. If your drive doesn't have one, you can measure bus voltage ripple with a scope during a heavy forming stroke. More than 5% ripple on a drive that used to be clean = caps are going.
Drive cooling fans are the other silent killer. They're usually small axial fans rated for 40,000-60,000 hours — sounds like a lot but that's 5-7 years of two-shift operation. When they slow down the drive thermal derating kicks in and you lose peak torque capacity. We replace all drive fans at the 5-year mark regardless. $15-40 per fan vs a $4,000 drive replacement is easy math.
One more: check your cabinet filter mats. A clogged filter mat can raise cabinet temp 10°C even with the cooler running. We cut them to size from bulk rolls — way cheaper than the OEM filters — and swap them monthly in our dusty stamping environment.