You can't see who gave you karma, and you can't see why or which post of yours was "karmised", as you say.
I guess karma helps you build a good reputation on this forum, but as others have pointed out before it isn't that reliable if your karma can only grow bigger and not smaller.
You also can't see other members karma development or filter post on awarded karma.
Other form software that use (a hack that allows) karma, can show which of your posts were rewarded with karma, but not for others to see yours, as far as i've seen yet.
If you want someone to know he did well, you'd tell him yourself you upped her/his karma by showing a + sign or just plain telling him they got some karma.