You find the expert, pay the expert, and hope the expert figures out what's going on. If your seller warrants Y2K compliance, be sure your purchase agreement contains a provision that warranties will survive closing. Without such a provision, warranties usually merge with the deed and lose their effect after your purchase is complete. Be sure you have survival language if you need it. Finally, you can always try to insure against Y2K losses.
Policies are out there, but they are rare, untested and expensive.
Talk to your broker. Ordinary business coverages may not help much. Such policies insure against accidental losses. Since people knew about Y2K long ago, it was foreseeable and expected and thus not an accident. Information of this sort isn't very well collected.