These lens all have a sweet spot of focus. Get a friend and t-shirt that has decent sized lettering and a football field. On a very bright day (10am-2pm, normally the worst time to take photos) have them start out about 35 yards away and walk away from you five yard at a time. Each five yards take a photo at optimum exposure. If it is a white t-shirt you may have to bracket the images. You will quickly find out where the lens will produce the best and sharpest image.

PJ's post about using the camera's metering is definitly something you should take note of in your testing. I've got a couple of older long lens that don't meter quite like I think they should for some odd optics reason, probably due to an anomaly or optical flaw that causes light to get more concentrated on one area of the TTL metering. Try the narrowest form, like spot or central metering with this lens. The light falloff at the corners on all the mirrors is dreadful and a matrix or average metering can easily produce bad results.

Post some images when you can, always like to see long distance shots.