If you're on PC, you can use a console command to see what your regeneration rate is. Just type player.getav magickaratemult Normal is 100. Anything above or below this value will be the percentage added/subtracted.
Yes, the numbers displayed are literally added or subtracted, for example if you have 150% magicka regen and you get another 50%, you won't get 150*1.5=225, you will get 150+50=200.
Yes, the numbers displayed are literally added or subtracted, for example if you have 150% magicka regen and you get another 50%, you won't get 150*1.5=225, you will get 150+50=200.