It is essential to know what the genjutsu does. Really though you kind of have to look at the perspective of both an actual human being reading a text and make sense of it while pretending the things that are actually happening with your bio / character.
You have to look it at from both sides. While doing that you must also look for key words in the jutsu, that kind of lets you know that it's a genjutsu. You use those key words to identifying it without meta gaming during a battle. So what are these key words? The key words in a jutsu basically tells you the flaw of the jutsu itself. Every genjutsu has one, some are more subtle than the other and you might have to read the sentence or the jutsu as a segment. You essentially going to have to break the jutsu down and figure out what it is actually doing and find where in the description tells you that there is a flaw. Some are not going to be flaws but a timing based where they might have to do something before the illusion takes place. You are really just cutting them off but you also have to make sure you don't break any time frame of sorts. There are some jutsu's that are dormant, and only activates when you actually trigger it to happen. In either case you'll have to let the genjutsu happen in some shape or form in order to know it's a genjutsu. Without it you won't know.
It is imparative you deduct a jutsu as a genjutsu correctly, I mentioned key words but if you screw it up then well.... you get screwed. So it's not that hard but practice makes perfect. With that said lets take a look at the first tecchnique. In each jutsu that I post you will deduct that it's a genjutsu. Tell me what the key words you think they are and why.
( Magen: Kokoni Arazu no Jutsu ) - Demonic Illusion: False Surroundings Technique
Rank: D
Type: Supplementary
Range: Short-Mid
Chakra Cost: 10 (+5 if you use it double)
Damage Points: N/A
Description: The user casts an illusion over an area that allows him to manipulate the way an area looks to fool an opponent. He can only change the way the terrain looks, and not any other feature.
