The quickest answer is everything prior to 1923 is in the public domain. There was no copyright law.
Next, anything from 1923 to 1963 for which copyright was not renewed.
Then, anything that was put directly into public domain by the originator.
And anything for which copyright laws do not apply.
For the rest, do some research. Public domain is one of the easier questions to resolve. Here's a start.
http://en.wikipedia.org/wiki/Public_domain
http://www.teachingcopyright.org/han...lic-domain-faq
http://fairuse.stanford.edu/overview...omain/welcome/
A broader view of that last reference, covering NOLOs total copyright section, is here:
http://fairuse.stanford.edu/overview/
Once you have all that info, start tracking down the first use of the block you have in mind. Try the MSU Quilt Index for samples, many are dated, of old quilts. There are also several websites that detail historical blocks and several well known books as well.
http://www.quiltindex.org/