John, I recommend you do a "brick wall test". What you would do is put your camera on a tripod, and then either use the camera's self timer or a cable release to take the photos. Doing so reduces the chances of any movement from tripod shake. In this case, take photos of the brick wall at 24, 35, 50 and 70, and at f 2.8 and f8 for each of these. Afterward you should open them up in Canon's software to verfiy where your specific focus point was. If the lens is in good shape, you shouldn't notice any softness at any of the focal lengths, though the 24-70 is sharper at 24, than at 70. The 24-70 should give you lots of detail, but you will notice more detail at f8, than at f2.8. After looking at the photos in Canon's software, open them up in Photoshop or a similar program and zoom to 100%, 200% and 400%. The sharpness will decrease as you zoom, which is perfectly normal, so long as you could detect sharpeness at 100%.
Some things to consider when evaluating your lens:
1. Shooting a white subject is likely to not give you the best focus a lens is capable of. Look for something more neutral like grey (or the bricks on a wall!)
2. If it is windy on the day you want to take your brick wall test, you need to wait for a different day. Even if you set your camera on a high-end tripod, you can watch the camera shake as the wind blows.
3. If you aren't familiar with how to read MTF charts, check out Michael's tutorial on how to do so. If necessary, you can search luminous-landscape to find the tutorial.