run your favorite games with msi afterburner and rivatuner statistics server on*
make sure you have both cpu usage and gpu usage showing
rivatuner is bundled with afterburner
if cpu is very high and gpu is low, you need coffee lake
but only if all cores are high. well it depends. if you have like 6th or 7th gen intel and only 1 core is high usage, then coffee lake wouldnt help much cause the IPCs didnt go up. but if you have all cores high usage then coffee lake will probably help cause it has more cores
if your gpu usage is high and cpu usage is low:
it's better you spend money on a new gpu rather than a new cpu and motherboard
At what point do I stop upgrading?
I'd say when you're comfortably above your monitor's refresh rate at the native resolution