How do I know if I'm going nearsighted?

The best thing is to book an appointment with an ophthalmologist or go to an optician to get your vision tested, because only they can confirm whether it's myopia or something else. What you're describing (difficulty seeing far away but not up close) is typically myopia, but there could be other reasons - eye strain, temporary focus problems, etc. It's nothing abnormal to start noticing this now; lots of people develop myopia at some point in their lives, especially if you spend a lot of time looking at screens. Either way, a quick appointment will clear up your doubt, and if it is myopia, glasses will sort it out right away.

The signs you're describing are indeed typical of developing myopia, but keep in mind that it can develop slowly and you might not even end up needing glasses for everything - lots of people only wear them for distance vision and leave them on their motorbike or in their bag. The important thing is to actually go to an ophthalmologist or optician to confirm it, because there are other things that can cause similar symptoms (eye strain, for example) and only a proper test will clear that up, but mainly because if it really is myopia, the earlier you catch it the better it is for controlling its progression.

True, everyone's already told you to go see an eye doctor (and you really do need to), but one thing nobody mentions is that you can do a quick home test: try covering one eye at a time and see which one has more difficulty, or if both have the same problem. That already tells you if it's symmetric myopia or if one side is worse - information you can bring to the doctor and speed up the diagnosis.

Now, just so you don't get false hope: that kind of difficulty seeing far away is usually myopia, especially if it started recently and is getting worse. The good news is that catching it early keeps your options open (glasses, contacts, surgery later if you want).