Are you wondering when a gear is suboptimal? Maybe you could have a better gear but you are not sure at what stats you should look for. Also, what is the most known max caps? (Example for % resistances, maximum life). If you want to know these questions, the following post you should view seriously.
Generally you just use life thresholds and have your resists maxed at all times (most importantly a6+). most people shoot for 1.2-1.4k life at a5 kitava, 2.8-3k at a10 kitava, then about 6k is a safe barrier for end mapping, but you can always (and should) shoot higher.
Resists 75% cap. The hard cap (not really needed is around 105%), meaning 75+30overcapped. Just for curses like ele weakness. Life, no cap. You can go for 200%-220% life on the tree, meaning at least 5k life, preferably 6k at around 90.
Typically on rare pieces you want to have life, 1-2 resistance rolls, and 0-1 offensive rolls. The amount of each stat that different gear types can roll depends on what kind of gear it is (helmet, gloves, boots, etc) and the item level. If you press alt when hovering over a gear piece you can see item level. The poe items level is the same as the level of the monster that dropped it, which you can check in the top right when your minimap is open.
When gearing a character your priorities should be: Mandatory uniques to make he build work. Stack life as much as possible. Armor and evasion are good to have and definitely make a difference, but you should prioritize high life values over a bit of extra evasion or armor.
If you have comfortable life (5.5-6k+) and capped resistances you can try to fill out whatever you have left with offensive stats. Typically the best slots for offensive stats are necklace and rings. Helmet, shield, and gloves can sometimes have a few useful offensive mods like attack speed, crit, or accuracy as well. Belts and boots are usually just utility.
The general elemental (fire,cold,lightning, chaos not included) resistance cap without taking modifiers to it in consideration is 75% you’ll lose 30 after act5 and another 30% after act 10 which would mean you will need 135% resistances from gear/tree to be capped in the endgame. There are certain mobs or maps (endgame areas) which can curse you with lvl 15 ‘elemental weakness’, which reduces your resistances by 34%, thus you need 169 resists from gear to be capped all the time.
After you killed kitava in act 5/10 the resistances get subtracted automatically, and are shown in your character sheet, that means you need 109% in your character sheet after finishing act 10.