For DKs, knowing your current attack power (AP) is very very important. As I mentioned in my last addon post, you want to pop your pets when your AP is as high as possible. If you run a rolling disease spec like I do (glyph of disease refreshes diseases with pestilence), then you want to start refreshing when your ap is as high as possible.
You can open your character window and watch your attack power rise and fall. You can do it that way, and you can fail, noob. :)
There are any number of addons that will let you monitor this important stat while in combat.
Tiny Melee Stats puts one to a few of your stats in text on your screen, wherever you want it. I have it at the bottom of my screen, next to my chat box. I can glance down there to see what my current ap is. I've found this is more useful to check my ap pre-pull; I can look at it and instantly know if I'm missing any buffs. It also displays my record attack power (currently just over 12k ap). It's nice, but in combat a little hard to see.
Bleeding Hearts is a very nice addon that puts a thermometer-looking graphic on your screen. When the thermometer is full, it's at your record ap. Try to roll diseases or pop gargoyle/dancing rune weapon when the thermometer is full. My first full raid with it last night we did some Ulduar with XT hard mode. That thermometer was fantastic! One thing to be aware of is that it only shows up on screen in a raid - not even in a five man, so you may think it's not working, but it is.
I had blood plague ticking for 750-3300, and critting for 1500-7600! Frost fever was very close behind. The higher your ap is when you start rolling diseases, the more insane your damage will be... so nice!