Determining the ideal resolution automatically?

Some of the images I have (some photos, some scans) are slightly
blurred, some more, some less. So, I can reduce the resolution
practically without loss of information. To be sure, I typically
reduce the resolution, then increase the resolution back to the
original and if there is no visible loss of information compared to
the original image I know that the resolution was not too low. It
typically takes a few attempts to find out how far I can go.

My question is: is there a tool that does this job automatically? I
mean, in a blurry image, determining what the "real" resolution is and
then downsample automatically? Sorry if this has been asked before, I
couldn't find anything.

Thanks!
Tim