what sort of evidence they used in this reconstruction?

Check the intro to Vox Latina by W. Sidney Allen. In the intro he answers both burning question: How do we know? and Who gives a flying fiddle-dee-dee? The two points I remember in answer to the first question are poetry and the rantings of prescrips. See, y'all *do have some redeeming social value. It just won't be realized for a thousand years or more.
http://www.amazon.com/exec/obidos/ASIN/0521379369/002-5183401-0038439