How to tell if heatsink needs replacing?

I’ve never heard any sloshing ;-}

There is a few ways to tell if the heat sink is failing even before you open the system!

Monitoring the thermal sensors using a good monitoring app like TG Pro I monitor the core temps and the Heatpipe outgoing air or Heatsink Proximity looking at the delta difference between them no more than 20℃. By far this is the easiest and works across all systems.

A good inferred viewer will show you the heat map between the heat sink to the fin area. That works on a MacBook Pro’s but you can’t effectively see the iMac’s as the CPU/GPU’s are on the back side.

If you have the heat sink out I use a scale to measure the weight of the heatsink comparing to a second (exact same model and both clean)

Looking at the logic board and the heat tube you might see a white power or staining if you suspect the system is over heating. Inspecting the heat pipe you might see a pinhole.