"You never really know with these things."
I'm going to borrow something from Liber E vel Exercitiorum;
Take a pack of (78) Tarot playing cards. Shuffle; cut. Draw one card. Without looking at it, try to name it. Write down the card you name, and the actual card. Repeat, and tabulate results.
This experiment is probably easier with an old genuine pack of Tarot cards, preferably a pack used for divination by some one who really understood the matter.
Remember that one should expect to name the right card once in 78 times. Also be careful to exclude all possibilities of obtaining the knowledge through the ordinary senses of sight and touch, or even smell.
There was once a man whose finger-tips were so sensitive that he could feel the shape and position of the pips, and so judge the card correctly.
It is better to try first, the easier form of the experiment, by guessing only the suit.
Remember that in 78 experiments you should obtain 22 trumps and 14 of each other suit; so that, without any clairvoyance at all, you can guess right twice in 7 times (roughly) by calling trumps each time.
Note that some cards are harmonious.
Thus it would not be a bad error to call the five of Swords (?The Lord of Defeat?) instead of the ten of Swords (?The Lord of Ruin?). But to call the Lord of Love (2 Cups) for the Lord of Strife (5 Wands) would show that you were getting nothing right.
Similarly, a card ruled by Mars would be harmonious with a 5, a card of Gemini with ?The Lovers.?
These harmonies must be thoroughly learnt, according to the numerous tables given in 777.
As you progress, you will find that you are able to distinguish the suit correctly three times in four and that very few indeed inharmonious errors occur, while in 78 experiments you are able to name the card aright as many as 15 or 20 times.
The point of all that is this; Crowley lays out the basic probability of the magical experiment, namely that without any clairvoyance you will guess correctly around seven times in the entire deck. The first seven or so successes you have can be chalked up to pure, blind probability. Every success you have after that suggests that it is not mere probability though. Each individual success is worth little - guessing right ten times may be a fluke, but that is less likely of 15 successes, and even less likely of 20, and so on. That is, every time you succeed it becomes less probable that it is random chance and more probable that there is some uniting, causal factor.
You can't be absolutely positive, but through repeated attempts and careful record keeping you can actually be remarkably confident about the results of your rituals.