Every lens has a point where it is sharpest, and a few points where it is soft. I use some of the very best lenses currently made, yet I still test every lens as soon as I get it to find out its sweet spot.
Here is how I go about it:
1. You will need: a tripod, your camera, the lens you want to check and a brick wall.
2. Set up looking straight at the wall. I use the joins in the brick work to check the sharpness.
3. If the lens is a zoom (let's say a 24-70mm) I will start at 24mm. Make sure it is in focus.
4. If you have been using auto focus, turn it to manual so it cannot change during the test.
5. Shoot one image at each aperture - f2.8, f3.5 etc all the way to f22.
6. Change the focal length to say 35mm (it's up to you) and repeat the shots.
7. Adjust to 50mm and repeat. Then finally to 70mm and do the same series of shots.
8. Upload the images to your computer. If you have a digital camera it will have recorded your settings in the file info. You will be able to see what the settings were for each shot. E.g. 35mm focal length at f9.
9. View the image at 100% and look at the edges of the image. This is where you will get the most noticable sharpness drop off. You will generally see that each lens has an f stop that is better than the others. This can even vary at different focal lengths.
10. Record the sharpest settings and write them down on a card that you can keep in your camera bag. I know professional photographers who will put a very small sticker on each lens that might say f14. This is a great reminder when you use that lens to try and work around its sweet spot. It may not always be possible to use the sweet spot because you need more light or you are chasing a particular effect like getting motion in water. At least you will know the sweet spot and be able to use it to improve the sharpness of your images whenever possible.
I would estimate on some of my best lenses the difference can be as much as 10% sharper.
I hope this helps, get out there and have fun - Steve