How many PCs and more importantly, how many resultant harddrives?
Your degree of certainty is a flexible animal: are you looking for rudimentary evidence or evidence that would stand up in a Court of Law?
If you consult a Forensic Data Recovery company, it will cost you a lot of money but you will be asked to be far more specific as to what you are looking for, otherwise the search (and resultant cost) could be prohibitive.
The file shredding / erasing software you mention is designed for the sole purpose of evading detection so, depending on how good it was, your search may prove to be in vain.
As for an easy way to do this, Data Recovery is never easy but usually costly, and given your criteria I think it may be pointless.