Practice is essential to anything in life that you want to be the best at. World of Warcraft is no different. I think it’s likely the folks at Blizzard realize this as well, since they did take the time to create practice dummies. These can be found in many cities, and I have provided you with links below.
If you find these useful then you might also find it useful to know that I have heard that these dummies do have a “back” side. This means that if you bring your melee character to one of these, you can test your damage from the front and back, as they should vary. For casters front and back are irrelevant.
In order to perform a quantifiable test you will need to know how much damage you do over a given period of time. You might also want to know your rate of damage per second or “dps”. There are a number of damage tracking addons that you could use but I prefer Recount.
There are a number of different things you can use Recount to tally, but for purposes of practice I generally use the “Damage Done” or “DPS” display. Clicking on the right arrow will change the information shown in the window. As you engage in attacking any target Recount will keep track of your DPS until you become “out of combat”. The next time you begin attacking it will track the DPS as a new instance. The most recent instance displays in the main window. To view past instances you can click on the page icon (not shown in the picture above, but it would look like the page crossed out in the picture, just not crossed out) and scroll down to the particular instance you’re interested in.
Now that you have something to keep track of your damage make your way out to the target dummy and try out some combinations.