How to identify the source of cooling problems?

One of our cars (a 528E) is showing in increase of heat. Or is it a decrease in cooling ability?? It runs hotter than previously. The change was sudden in its onset.

I replaced the thermostat and that made for no real difference. The car not too long ago received a new water pump. How to tell if the problem is due to the radiator? Or perhaps the clutch on the fan?? Or ???

Any input appreciated

TIA

...Tracy

__________________
...Tracy

'00 ML320 "Casper"
'92 400E "Stella"