And is there a way to know what level any given item is? Not directly. As a rule of thumb, the top level you get will be either something of the best type with either no enchantment or a poor one (e.g. an unenchanted Duelist's Blade), or an item of no better than average type with a good enchantment (e.g. a Vampiric Battleaxe).

And items of * skill cannot stack, except with artifacts/relics, so you may only get a bonus from one artifact/relic and one from a regular item, correct? Again, correct.

If so, what is the hero's belt treated like? Neither. In MM6, there were only artifacts and relics. In this game, there is another class, called (rather clumsily) "Specialty Items". Hero's Belt is one of those. I can't remember for certain what it stacks with, but I would imagine that it would stack with an item "of Armsmastery", and the rest of its attributes are either cumulative anyway (+ 15 Might, of Protection), or stacking doesn't apply (of Regeneration).