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I'm going to upgrade my PC soon. I'm worried that I will no longer spot performance losses in my game because of the better specs.
I can check memory usage easily, but how do I check and debug CPU and GPU usage? I used to rely on the framerate, but I guess that it is not accurate.
They're called profilers. Visual Studio has both CPU and GPU profilers built in to recent versions. A profiler will give you an idea of how much time your app is taking and where that time is being spent. This won't entirely avert the problems of over-specced hardware, though; the app might peg a lesser CPU but only use 60% of your developer machine. They can help you track trends, e.g. noticing that what used to take 0.5ms now takes 2ms, which is indicative of changes that broke performance.
Frame rate has never been useful for performance measuring. You need to time things. You can build an in-game instrumented profiler, which can be a huge help (and companies like RAD sell some off-the-shelf instrumented profilers geared for games) or you can use a separate sampling profiler which collects stats from a running program without needing to modify it.
Tools like Very Sleep are free profilers that you can run if you're not in Visual Studio or don't want to pay for more competent developer tools (which are often a far better investments than new hardware, I might note). There are similar tools available for OSX, Linux, consoles, and the mobile platforms. Intel's VTune is the most capable sampling profiler I know of, but it is expensive and only works on Intel CPUs naturally (though it does support non-Windows OSes).
For GPU, you can use stand-alone tools like NVIDIA's NSight or AMD's PerfStudio or Intel's GPA. Again, there are some tools for other platforms besides Windows; the ones for consoles tend to be top-notch while the ones for Linux are currently all junk and I'm not personally familiar with the offerings on other platforms.
Ultimately, though, you need to test on target hardware. Figure out what your target low-end hardware is and then buy one of those (or find a tester that has one). Test on that. There's absolutely no other good way to ensure that your game will run well on that hardware other than to just test it with that specific hardware. This is true for PCs, mobile devices, etc. (and this is one of the reasons many developers prefer targetting consoles or closed ecosystems like Apple's products; it vastly reduces the number of hardware configurations that need to be tested)
Game development studios will often have whole labs of varying configurations (different levels of performance, hardware from different manufacturers, different versions of the target OS, etc.) to do their testing. Smaller indie devs might have a couple machines to test on, like a primary desktop, a backup desktop, and a laptop (and then have to rely far more on volunteer testing).
