The modified_date in sys.objects would reflect the last change to the index. That might be as close as you can get if you don't have some type of monitoring set up to capture those types of changes. Not sure what changes can be done on a disabled index, if any. Most that I can think of require the index to be enabled.
An index change would be picked up in the default trace if it was known as a recent change. I don't think it tells you more than the object was altered though.
Sue