>I saw a demo lock solution that was simple and worked something like this:
>A file with an expiration date in a character field, with the format:
>

>cExpireDate="08.01.2001"
>if !"."$cExpireDate
>   * quit
>endif
>
>When the demo opens it determines that the "dots" are still in place, then;
>dExpireDate=ctod(strtran(cExpireDate,".","/"))
>if dExpireDate>=date()
>   repl cExpireDate with strtran(cExpireDate,".","/")
>endif
>subsequent re-opens will notice the dots have been replaced with slashes, and
>not allow the demo to run. I someone hacks the Exiration date field, they
>will most likely leave the dashes in place, which means the demo will not
>execute

In this case, the "." vs "/" could have been placed anywhere, even the windows registry, all by itself as a simple flag. 

If it's in a file local to the app, and assuming one does backups, or creates a duplicate install, a file compare could reveal what had changed. Some utilities will compare the contents of 2 directories (files and file contents), highlighting all the differences.

But for most cases, it may still be a good solution; one actually winds up looking at the solution, but can't figure out the problem (or vise versa).