First of all, make sure it is sharp somewhere.
I think no-one would usualy recommend this for checking focus of a lens, but, it is valid for checking for sharpness.
Get a tablecloth, or sofa cushion, something that is flat, and put a ruler down on it horizontaly, point left to right around half-way down it.
Now set your camera to Av mode, and adjust your ISO (or even use flash) until you get at least 1/100th of a second.
Now, at a distance of around 1m, at an angle of around 45 degress use the autofocus to focus on the ruler.
Now pixel peep at the image.
We are not checking for focus accuracy. This is not a test for that. Angled targets are not ideal for that.
However, what you should find, is a horizontal band in your image, where the fabric is lovely and sharp.
(However, on the 400d I had, ISO handling wasn't great, so maybe try not to go above 400 ISO?)
If the sharpness band is not horizontal, but diagonal, it either means that you were not square on, or there is a problem with the lens.
If it is horizontal, but doesn't meet the ruler (remember this strictly wasn't an autofocus test), then either the lens/camera is front/back focusing, or the fact that the camera uses a vertical line of sensors is meaning that the focus was of, or, if you are like me, you took a breath in between focusing and pressing go.
The depth of field is very shallow, a cm could be enough to through focus off.
Edit:
I have just done this as a test myself. Here are two 100% crops. As you can see, the area of focus is less than the width of the ruler at f/2.0, so you need to be sure of where you are focusing.
One is focused on the ruler, and then the other is focused on the material itself (with a bit of grass for help).
(the ruler is actually somewhat sharper on my screen, but saving at jpeg level 4 to get it under 200k makes things more blury)
