detect stderr writes when combined

[Robin Becker]

This is possibly a stupid question, but I would like to have a particular sh 
script stdout and stderr combined, but at least detect when stderr has been used.

In particular for my cron scripts it seems that error messages get wrapped up 
and emailed, but they are then out of context with the normal informative 
output. If I could have the combined output go to my logs in the normal way and 
detect that some error output had occurred I could then email the whole cron output.

I'm fairly sure this is doable with some C, but is there an easier way?
-- 
Robin Becker