I haven't looked on the nexus for this, so assuming there isn't a damage meter mod available, you can use console to test. This might seem a bit labor intensive, but it works.
First, always, make a hard save, so you can revert if something goes wrong.
Go someplace with relatively few enemies, like Redoran's Retreat. Have your follower(s) wait outside. Use console command tgm, to go into god mode, if need be.
Kill all but one enemy, probably leaving you the bandit chief. Point at him, open console with ~, and click on him with the console mouse until you see his ID number beneath your cross hairs.
Type the command...
setav health 1000
That will set the actor value health to 1000, so you have a known and easily calculable number to work with. Don't worry. You're still in god mode.
Close console and hit him once, without using weakness to magic. Reopen console, make sure his ID number is still visible, and type...
getav health
...and see what his numbers are. Let's say he's now 950, you know you did 50 damage. Set his health to 1000 again, close console, apply your weakness to magicka effect, and use the same attack.
getav health
...again, and see what the numbers are that time. Do this a few times, to make sure you're getting consistent results, and aren't factoring in a harder hit than the others.
Then, when you know whether you're hurting him more, repeat the same experiments using
player.getav health
to see where your numbers go, when you hit him with and without the effect. You may need to turn off god mode momentarily, let him hit you, check your numbers and go back into god mode, etc.
It's a bit tedious, but it has helped me figure out actual damage, in the past.
