How can I detect an invalid date string in SQL?

In SQL, how can I detect a NULL returned by a function like DATE() or
DEC()?

This is my statement:

select storagedt, ifnull(dec(date(storagedt)), 0) as "as Decimal" from
mytable

storagedt is a VARCHAR(64)

here is the result in SQuirreL Client:

STORAGEDT as Decimal
-------------------------------------------------------------------
1993-12-02 19,931,202
RK <null>
RK <null>
RK <null>
RK <null>
CS <null>
CS <null>
RK <null>
1994-05-11 19,940,511
RK - IC <null>
1994-01-26 19,940,126

Apparently an invalid date does not return a real null, but how can I
detect that? I know that iDate is available, but I don't really have
install authority on IBM i.

Run SQL Statements in iACS shows a -- instead of <null>, and STRSQL
displays +++++++++++++++.