This is easy to check in the CS but can also be tested in-game by placing items in mod containers, travelling to another cell, and resting for three days. Unsafe containers will have respawned and eaten your items.
A suprising number of house mods (including some popular mods) have unsafe storage. So... does anyone know if there's a list of house mods with verified safe storage? If not, how about we make one? I'd like to find a second house for my current character, and it'd be nice to have a list of houses known to be safe.
I'll start: I know that all storage in http://planetelderscrolls.gamespy.com/View.php?view=OblivionMods.Detail&id=5805 is safe because I created it.