Can you clarify what you mean by that question?
Also, take a look at future/furrr packages that make using parallelization much easier. future is an "interface of interfaces", so you can use it with snow, parallel, mclapply, slurm etc.
Most of the time you'll see something like availableCores or detectCores or something like that in the code, but there is no way to know for sure how many cores are used by any given function. But can you clarify why that is important for you to know?