Include custom enhancements or add-in modules. Check the vendors' Web sites for Y2K info. Enter sample transactions for late 1999 and early 2000. Run the transactions through their full cycle and assess the outcome of each. Consider delaying any significant IS projects during late 1999 and early 2000 so that your IS staff can deal with any unexpected problems.
Questionnaires are helpful, but for key partners, personal contact is preferred.