You're right, this is fairly system dependent - However, basic database metadata can be determined through the metadata attributes of DBI, specifically the NAME, TYPE, PRECISION and NULLABLE field attributes. The standard values for common SQL data field types returned by the TYPE attribute are as follows:

SQL_CHAR              1
SQL_NUMERIC           2
SQL_DECIMAL           3
SQL_INTEGER           4
SQL_SMALLINT          5
SQL_FLOAT             6
SQL_REAL              7
SQL_DOUBLE            8
SQL_DATE              9
SQL_TIME             10
SQL_TIMESTAMP        11
SQL_VARCHAR          12
SQL_LONGVARCHAR      -1
SQL_BINARY           -2
SQL_VARBINARY        -3
SQL_LONGVARBINARY    -4
SQL_BIGINT           -5
SQL_TINYINT          -6
SQL_BIT              -7
SQL_WCHAR            -8
SQL_WVARCHAR         -9
SQL_WLONGVARCHAR     -10
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This information can be retrieved from a DBI database handle similar that shown in one of my first SOPW questions to the monastery back here.

While these attributes allow information about the most standard of SQL field types to be determined, there may be some non-standard field types returned by databases with extended field types. From my own experience I find its usually better for just this reason to gleam an understanding of the database structure directly and incorporate this understanding into my code, rather than trying to have the code determine this information by itself.