If you know Improved Identify, you can cast it. There is apparently one item (though not common) that lets you identify items as well.

Detect Magic will tell you if items are magical, and True Seeing will additionally tell you if the items are cursed.

Otherwise, you need to experiment:

For wands, try using them. That may reveal the effects after one use. (I believe wands do have limited charges, however.)
Potions are often identified after drinking one. Note that, in any given playthrough, all potions with the same description are of the same type. (This also applies to wands and rings.)
For rings, experiment. Some rings have visible effects on your stats. Otherwise, try wearing the ring and seeing if you notice anything different.

You can find mirror (it is pickable item) somewhere on dungeon floor. You can use it (in right hand) to identify item (in left hand). It has limited amount of charges though. So of you'll find mirror, I'd recommend saving, then IDing items (to know what they are), then reloading.

Unless you are playing no-reload (which I recommend if you are playing with Death Real enabled). In this case, you should prefer identifying items that could be dangerous, or, if you have True Seeing (maybe you're a Cleric), weapons so you know their bonuses.

Armor bonuses can be easily determined by equipping them and checking your AC.

Thanks.....your replies are very helpful. Ps. I just found the mirror.

Bracers of Defenselessness? Nah, no way :))

If you have True Seeing (Clerics learn it but not Improved Identify), cast that spell and cursed items should glow red. Also works for rings and other cursed items. (Not sure about potions, however.)