Easiest way to check is by testing at different settings.
Run at the highest resolution your monitor can do, check the performance.
Then run at the lowest you can set, if performance increases, GPU should be the limiting factor. If it stays the same, the limit has to be somewhere else.
Or you can obviously check the load. Optimally the GPU should be sitting at 99%+ while gaming, if it isn't its held back by something else.
What can hold it back? Many things.
- CPU can be loaded (total CPU load, but also individual cores)
- RAM can be full
- Memory on the GPU can be full
- Game might have trouble loading stuff from disk
- In online games the connection might cause it
- CPU or GPU might throttle down from temperature or power draw
Also worth running some benchmarks and comparing your result to other people's on the same hardware. Yours should be within maybe 5-10% of others (slight differences from different temps, oc, background tasks etc. But it should be somewhat close)
So basically:
- - test the game at different settings
- - check that temperatures and clocks are what they're supposed to. (GPU should boost to around 1850-1900 MHz, CPU around 4.0.-4.2 GHz, depending on what you do RAM obviously depends on which you have)
- - check the load of your parts
- - compare benchmark results to other people with the same hardware