If you not just wanna know the fps or the hardware monitoring (afterburner), but also if you're CPU or GPU limited, then CapFrameX is currently the best tool to use.
Intel created some really nice software while they were focusing on their graphics cards and it's free to implement into other software.
CapFrameX did this and it measures the time the CPU & GPU needed for each single frame.
Examples:
CPU = 6ms
GPU = 7.5ms
FPS Limiter at 90 fps => 1000ms/90 = 11.1ms per frame
Then your fps limiter is limiting the fps and your real limit would be at 7.5ms due to the graphics card, which equals 133.33 fps.
You can also see average, 1% low etc so you'll see if your fps drops were caused by some spike on CPU or GPU frametimes.
It's brilliant and quite easy to use.