The most important variable to monitor is threads_running. This
variable keeps track of the number of concurrent statements
currently executing in the MySQL Server.
If this variable has spikes that put it in a region where the
server won't operate optimally (usually going beyond 40 for
InnoDB workloads) and most particular if it goes well beyond
this into the hundreds or even thousands of concurrent
statements then the thread pool will be something beneficial
to protect the MySQL Server from troubles in overload
Another indicator that you will benefit from use of the thread
pool is when you already now use the --innodb-thread-concurrency
variable. This variable tries to solve a similar problem and the
thread pool solves it at a better place even before query execution
has started and also provides additional benefits.
Also if your workload is mainly short queries then the thread
pool will be beneficial, long queries isn't bad for the thread
pool but will decrease its positive impact.