How can I tell what level of difficulty I've completed on a given mission?

In Hitman Absolution, where do I look, or how can I tell which missions have been completed on a particular difficulty level? I played some missions on professional difficulty, and some on normal, but I can't remember which ones I've done on each difficulty setting. I'm trying to get the Absolution achievement, and I don't want to go through each one over again.

Short answer, you can't. In this case, having a good memory and/or having the foresight of noting down which level you've played at which difficulty is paramount.

You can, however, look at the total score you have for each level. If the score is higher than you'd expect to get from normal mode, than it's very likely that you have finished the level on hard. Remember, in normal mode you get 25% bonus to your total score and in hard mode, 50%. This could actually be quite reliable depending on your playing style. Because under normal mode, many players actually tend to just go all gun-ho on the enemies, whereas in hard mode, you are forced to take the steathier approach, as you take more damage, do less damage to enemies (or they have more hitpoints; irrevelant in this case); enemies are more alert and numerous. So ironically the more meticulous and careful you are when playing on normal mode, the less you are able to tell the differences in score between difficulties.