kernel activity meassurement
 

Where can I see the amount of CPU time the kernel is taking?

in top: is it the sys cpu time?

are there other ways to know, one way or another?

The system time in the process statistics is the amount of time your process spend in kernel mode (i.e. executing system calls). The sar command should help you and can also use vmstat and iostat to get some idea. There may be a better way too that I'm missing.