How can I find out if a parameter is a DB_KEY?

When describing "select column1 from testrowid where rdb$db_key = ?",
how can I derive from the parameter information that this is actually a
db_key?

For selected fields like in "select rdb$db_key, column1 from testrowid"
it is easy: just check if isc_info_sql_field is "DB_KEY", but this
doesn't work for parameters, because those don't include name information.

Mark
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Mark Rotteveel