To answer your original question: you can mainly tell by doing benchmarks and watching your CPU/GPU usage. If your CPU is maxed the whole time but your GPU is chilling at 50-60% usage while you're getting below 60 FPS, you likely have a CPU bottleneck. There are a number of free benchmarks out there, and several "AAA" games will typically have one too (Forza, Returnal, and many others) so you can tune your system.

So buying a 4080Ti without the supporting parts it needs will limit how much performance you can get out of it. Nowadays RAM typically is not the bottleneck.