Getting datatype before SELECT

In my C code which communicates with the Postgres database,
I have the need to determine the datatype of a column, before
I have done a SELECT command.  I have the name of the column,
but I cannot seem to figure out how to get the datatype information
until after I have done a SELECT.  Then I can call  PQfnumber() and
PQftype() to get the type.  Does anyone know how to do this?

Thanks,

Glenn