Canon publishes a set of MTF curves for most of their lenses. These are not measured curves, but curves plotted by computer which show what a perfect lens of that design could do. Manufactureing tolerances are certain to guarantee that no perfect lens exists.
Otherwise, Photozone, DXO, and a host of others measure the resolution of a camera - lens combination. The results are valid for only that combination of Camera body and lens, and even then, there is a lot of variation.
So, read all the test reviews, paying attention to what body was used, and you will at least have a reasonable idea of performance to expect.