It can be very hard to say. I was having trouble with BF1 at times and did not really know if it was my CPU or GPU. There was a marked difference between my 4 core i5 at stock and overclocked, but I was also maxing out my GPU.
I just did the platform upgrade and now have a 2600X. The difference is huge! Only the most demanding parts of the nicest maps cause slowdown now and it is still very playable. I actually turned up a few graphics settings and plan on keeping my video card a good bit longer now.
BF1 with 64 player maps is notoriously CPU intensive though. But, I am also running at 1440p, so graphics were stressed.
Truth be told, if I really wanted to find out, I could have ran at say 50% resolution and see if I still had slowdown. I guess what I am trying to say is that if you have a fairly matched CPU/GPU, it can be both, but maybe one more than the other.