You can check all the affixes and their quality at GrimTools. I also never know if an item is double rare or not. But you can enhance the affix display by using the Item Color Filter. This tool colours the afixes regarding their quality. This makes it easy to distinguish double rares.
Oh yea it could be the case that this item is a double rare MI. But I dont know the quality of the affixes by hearth.
A double rare is any item with 2 rare affixes (e.g Stonehide… Of Kings); for the most part a double rare is referred to as an MI with 2 rare affixes (e.g Thunderstruck kupacabra’s Chausses of the Flesh Hulk)
Something like “Stonehide… Of spellweaving” is not considered a double rare due to the suffix in this case being a magic affix rather than a rare one
Allow me to be pedantic 
MI with double rares are called “double rare MI’s”
For example, stoneplate shoes are very cheap to craft, and are most certainly not an MI, but you’ll often see them referred to as double rares.
EDIT: however, getting a double rare stoneplate is extremely difficult and…well. Rare.
Ok, so there are “rare” affixes, learning new stuff here.
Now question is, are MI items only worth picking up if they are double rare?.
(just trying to get a general rule on what’s worth picking up).
That depends… on you. On your build. And what’s useful to your build(s) at any given point in time.
I dont care about my current build, im picking up items for any potential future build, and I would like to pickup rare green items if I see one.
Then that only broadens the scope of the qualification to what you “think” might be useful for characters down the line. If you know what you have in mind then you might have a decent idea of the affixes you might deem desirable. Overall it just becomes a matter of knowing what you intend and learning which affixes would be the “optimal” choices.
It’s hard to find a formula to know what’s good and whats bad. I guess thats part of what makes this game more interesting than others in the genre, but at the same time it makes it a bit unappealing for non-praetorians 
#praetorianprivilege
Don’t know about Praetorian privilege, more just about learning what items are good and what items aren’t which anyone can do. 
That’s actually NOT a Praetorian-privilege… it’s an “anyone who has played the game enough to know those sorts of things” privilege.
FYI - in my case I really have never bothered to go out of my way to “learn” which affixes are which. That is just something that comes naturally over time.
Easy to say for you since you are praetorian 
Just kidding man, a bit of sense of humor here
But we all had to learn stuff about the game, otherwise none of us would be Praets. It’s just some of us learnt more than others. 
As powbam said, I’ve never made a point of learning which affixes are good or not. I just pick it up as I go along and I’m pretty sure I still don’t know most of them. 
It’s all good - we just tend to get a little sensitive to any portrayal that might imply we are somehow better than anyone else on the forum 
There is naturally some “nice” privileges that do come with it by proxy but never mistake the fact such privilege is tied to the whim of Our Lord and Savior, Zantai. He giveth, he can taketh away.
The knowledge in this game comes naturally… after 5k+ hours i suppose 
To speed up the process you need to 1) explore the grimtoools site and everything it has to offer 2) dig into some guides on beginner builds and game mechanics, specifically this one is very thorough and well written: 🤯 malawiglenn’s guide on game mechanics for beginners
Here’s the ez summary to affixes and MI’s:
Look at the base skill modifier on the MI (if any)
Do the affixes associated give relevant stats to said skill?
a. Yes: Awesome. Great drop
b: No: See 3.
Do the affixes give relevant stats to a damage type which the modified skill can be converted to?
a. Yes: Good affix for conversion builds. Keep
b. No: Vendor trash.
So the next thing we need to understand is…what are relevant affixes?
If you can find an MI with 2 of the above, it’s probably worth keeping.
If you still have no idea what to keep, horde all greens which sell for more than 5k bits.
Some examples:
https://www.grimtools.com/calc/a2dJmxdZ
On the player’s left is a non-rare MI, on the right is a double rare MI.
Which do you think is more relevant?
Oh then, the extreme tl;dr would be: more text = better!.
In this case, sure, quite often the “wordier” items do tend to “better”.