Calling is a better way to check references than writing because people will sometimes tell you information that they don't want to put in writing.
Whatever you ask, though, be sure it's job-related. You don't want to be accused of invading anyone's privacy or of discriminating against them. Here are some tips for successful over-the-phone reference checks:
The Business Tools area includes a sample script that you can use when you get on the telephone with a former employer.
If necessary, offer to fax the job applicant's signed authorization to release information