null date comparison

I'm having trouble doing a select on empty date fields.

Suppose my table is composed of employee_number(char), hire_date(date), and
termination_date(date).
I would expect to be able to say:

'select * from employee_tables where termination_date = null'
or:
'select * from employee_tables where isfinite(termination_date) <>
1'

...but no luck. What is the proper way to select undefined dates?