I agree with the above comments. You need to figure out what you've got, and then move through your process. Some guys are looking for a "set it to this and you'll be golden" type of information, while others want additionally to know why, and still others (i.e., yours truly) are more down to experiment with a lot of (or all the) settings so they know for themselves what's what. Whichever approach you take, you'll first need to be able to say what you have in the bike.
I'll add some links that with information about rejetting. Seems there are a variety of opinions on the topic. Since you're asking about "how do I know when it's right," they may be helpful (or not).
Here's one from FactoryPro.
This is a personal favorite of mine, as it provides a routine that (a) is focused on a particular goal [power], (b) feels rational in that it implies an awareness and adjustment to every part of the carb circuit, and (c) feels comprehensive in that it provides a number of ways to "judge" or "sense" richness or leanness.
Reading this got me down the road of paying attention to the overlap between the idle, pilot, needle and main circuits, and also made me aware of the potential to use engine temperature (cold, operating, hot) to gain insight about what was happening with the mix.
The overwhelming emphasis on power implied by the routine also started me thinking about the different motives in setting up the carb. Personally, I look for smooth feels and sure acceleration, which dovetails nicely with this guy's approach. Note: this surely comes at the expense of fuel economy.
Here's one from the "junkman."
This guys has a series of videos that, while junky in style, had some valuable background info about how carbs work.
Here's an interesting one from a guy who settles on a configuration well outside the norm.
Granted, his bike is all souped up and whatnot but it's worth the read, if not for specific information then at least for background.
And here's an instruction sheet from Schnitz racing that talks about the KLX needle.
Notice this quote: "A less restrictive exhaust system, and/or air box mods, may need a main jet in the #142.5 to 150 range. The stock main jet (#148 for 87 to 07, #145 for the 08+) often works well. It may also need the needle circlip in the #3 or 4 position."
But also pay attention to what it says about altitude.
I also found the comments interesting on Schnitz' page where they sell the KLX needle.
And lastly, here's a tech book.
Super informative, if you really want to go that far.