Not sure if this helps in your circumstance, but I had a similar
issue.

When attempting to post the record, I just stepped through each
field of the record (in a for loop) testing for .IsNull on each
field.
If all fields were null (except for the first two, as these are
gen_id primary key fields) I implicitly called a cancel, otherwise I
would attempt a post which would generate the necessary error if a
field was null when it wasn't allowed to be.

-Ryan