APEX

I have a conditional computation used to set the value of an application-level item.
In the condition, I need to check to see if the action (button-press to initiate DML processing) generated an error (eg cannot delete row because of child records). I don't need to catch/handle any specific errors, I just need to know if an error occurred.

What do I check for? I've seen references on the forum to hidden page items Pxx_msg and Pxx_err, but I've played around with them, and apparently don't know how they're set.

Another thought I had is to create a before-computation process that checks to see if there is some sort of error/exception, but I'm not having luck w/ this either.

Ideas?
Thanks,
C