Effects don't stack with themselves, and duplicate items don't stack their special effects (unless you abuse a bug). So two identical Staves of Elemental Mastery won't produce +2 to elements, but different ones will.
As for finding which items give identical effect with each other or a spell, check the manual. Marble Armor and Boots of Stone both have the effect "Stoneskin", so that won't stack with Stoneskin the spell.
I guess it's just me but I really like those extra life bottles. It's just takes away a lot of stress knowing that my carefully developed SCs need to be killed twice. And the anti-aging quality is a nice bonus as well.