I designed an application for a call centre where there was a strong requirement for prior client identification. I provided a series of fields for appropriate identifcaton (name, address, telephone, account number and so forth). A command button would populate a list box or combo box with all matching records. If there were none, a new form (or simply another tab page on the same form, if you like) allowed entry of new client information, otherwise selection from the list box simply proceeded to the business of the day (another form or tab page).
If it is useful and my client agrees, I might be able to make the material availale.