Try this.

=IF(FIND(",",A1)>1,"Dr. " & LEFT(A1,((FIND(",",A1)-1))))

It goes a step further and extracts the entire surname.

Otherwise, just use the find the command to identify the comma. Find returns the number of characters up to the search term (,). So if the result is > 0 then, you have found a doctor.

Cheers.