It seems like some people use divination to determine whether or not a given spell will “work”. Others seem to use it determine the unintended consequences of a spell that they assume will work but may lead to bad things. How do you all usually use it?
I ask my spirits, more rarely I use the “Book Of Fate”, note and info in that post on how to use it.
But usually I ask my spirits.
I don’t ever do divination to determine if a spell will work because I am so confident in my abilities and the power of the demons and familiars I work with that I know my spells will work.
I use divination to find out information about my future, what moves I should take to succeed, how to avoid something potentially bad, information on other people (only as needed), things like that. My familiars tell me things as well and my intuition has been twice as accurate lately so I haven’t needed to do much divination as most of the answers I seek either come to me naturally or a spirit tells me.
I hate the book of fate, I do not trust it, it’s unreliable.
This is a very important point here that, while mentioned by many practitioners from multiple paths, just is not stressed enough. If you go into a ritual or working with DOUBT then chances are it will not work. This could be construed as a self-affirming falacy, but some of the stories that I have read on here definetly throw the usual tenants of that out of the window.
Time and time again I have gone into a ritual or something similar with doubt in my mind and I got little to no results. Only when I have gone in with complete and utter certainty (dare I even say “faith,” or rather a virtue in the absurd) has my mind been blown away with what actually happened. When I set aside masks, set aside to be pretending to be so and so, and go in truthfully, honestly, and expecting the results 100% have they manifest, and sometimes in terrifying ways.
Recently I did a ritual to try and reconnect with my guardian demon/entity. I decided to go into it as I had before but without a lot of the facades and bs I put up. I went in completely as myself, laying myself bare and truthful. If something happened it happened, if it did not then it did not. I would not force something. And let me tell you, RIGHT as I got out of the ritual I was completely blasted with something I cannot explain. I was knocked down onto my bed and could barely stand; I was absolutely terrified. It was like I was fading in and out of realities, like something was trying to take over me. I can’t even fully remember it but it was the most real experience I have ever had. It was also the ritual where I went into it the most honest and truthful I have ever been. Go in honestly, do not lie or pretend, and the results will come. Believe me.
Largely due to the help of Lady Eva and RavensAscent and others on this forum, I am finally seeing some small, baby step accomplishments in magick. In the past I have asked, “What is a reasonable expectation of magick versus an unreasonable expectation?” Lady Eva was very helpful when she said something like, “Magick makes the improbably, but possible, very probably.” In other words, it stacks the decks heavily in your favor. So, under that model, healing cancer is possible because the body has a mechanism for it in place. However, growing back a limb would most likely not be possible because (at least at this time) there doesn’t appear to be a biological mechanism to accomplish that. Therefore, a spell for that would not work. However, there are a ton of grey areas I would think. My thought is that divination would help in determining such things. For example, E.A. once said that he initially hesitated to do rituals for hire for people because they were asking for ridiculous things. His summary was, “You have to work within reality. If you were caught red handed slaughtering your whole family, you are probably going to do some time. But we can help minimize the penalties.” Thoughts?
I don’t see it as being about doubt, “pass/fail” binaries, it’s more like the way I’d check the weather forecast before planning a picnic or outing.
Sometimes something is on its way to me that would be better (or more important for my empowerment) than the outcome of the spell, other times there are too many vested interests pushing against it, so me getting that outcome might do harm I wouldn’t want to someone else.
I’m in a major pact and I won’t do anything that would conflict with the big-picture outcomes of the spirit I’m pacted with, for example - this matters to me, because our desired outcomes have basically become aligned.
You wouldn’t take the only car in the household for a leisurely drive to the park, if you knew your spouse needed it to drive to a work presentation that was critical to their career, right? 
Obviously, there are some things I just intend to damned well have, and screw any forces that try to work against them - I’ve sometimes proceeded with a spell or other action because I was single-minded and didn’t intend to let the fact the cosmic weather was against me, matter a damn.
Sometimes that opposition is negotiable, and a third way that suits bioth parties can be found, but if you don’t know this in advance, you’re more likely to end up just being blocked or frustrated.
So even then, it comes in useful to know in advance about possible opposition, and its likely forms and reasons.
I also think divination may help to determine whether divination or mundane means would be the most efficient course of action.