As others have said, it’s really a matter of just knowing or waiting to see results manifest. Being patient for spellwork is one of the hardest parts of being a witch, in my opinion. It’s not like the movies or TV shows – we can’t just say an incantation and have immediate results, no matter how much we wish it were true. There are things you can do to help you be mindful of spell results, such as journaling your spell, but sometimes the “cast and wait” method works just as well. There’s a good passage in the book I just finished reading for 
Book Club XXXX about keeping track of your spells. It’s an exercise in journaling your spellwork.
Exercise 6 - Journalling Your Magic
One way to keep track of your results and work at your skills of discernment is to journal. This does not have to be in a large, fancy, leather-boudn tome which you call a Book of Shadows or any other mystical name. It can be if you want but focus more on the act of writing itself. A piece of paper and a pen, a document saved in your computer or whatever is most accessible to you will suffice. Writing in itself is a very spiritual process, transforming your thoughts and experiences into a physical form; it’s a very powerful and magical thing.
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Before casting the spell, perhaps jot down the following:
How long before you expect to see results from the spell?
How many people are involved in the situation and what are their names?
What type of result do you expect; positive, negative, any particular emotions from any particular parties?
If the spell does fail or not work out well, do you have any other spells or practices to employ to add to the magic?
After the spell and time passes, you may want to consider the following journalling prompts to consider engaging your magical thinking:
How have you felt since casting the spell?
Has the target (you or someone else) been acting differently that can’t be explained by other normal means?
Have there been any new recurring thoughts related to the spell since you casted it?
Have you been noticing any repeating symbols, words, sentences around you that can’t be explained by common circumstances?
Do you recall any noteworthy dreams that may be related to the spell?
– Spellcraft and Spellwork by Ariana Carrasca