How do I find out if an abbreviation is an initialism, which means I need to put "the" before it, or an acronym, for which I don't need to? (If I don't know how it is pronounced)
The only difference between an acronym and an initialism is how it is pronounced. If you don't know how it is pronounced, then you don't know if it an acronym. You may be able to find the abbreviation in a dictionary which will tell you the pronunciation. Note that there is often variation in actual use. You will hear "Lol (laugh out loud)" being read as ell-oh-ell and as /lol/.
If an abbreviation is written with periods "l.o.l." you can be sure that it is intended to be read as letters, and if it is unpronounceable "BBC", it is probably read as letters. But look out for examples like "SQL" (sometimes read as /sequel/, but you would not use "the" with SQL anyway, however it is read). The only way to know this is to learn it from a dictionary or from somebody who knows the pronunciation.
On the matter of articles: see why no article before G.D.P.? (an article is not always used before an initialism)
There are some abbreviations that look unpronounceable but are still sometimes pronounced as acronyms. The one that comes to my mind is SQL, pronounced either like “sequel” or as an initialism.– Laurel ♦Sep 25, 2020 at 22:33
@Laurel - And sometimes an abbreviation that looks pronounceable is still spelled out as an initialism. Names of countries, for example: USA and UK could be /yoosa/ and /uck/, but they never are. Sep 25, 2020 at 23:33
And see my first paragraph.... You have to know how it is pronounced. And my second, "the only way to know this is to learn it"– James KSep 25, 2020 at 23:34
We’re not even consistent. AIDS is pronounced but HIV isn’t, and neither takes an article. WHO isn’t pronounced, but does take an article. I sense there’s a rule here since I usually guess right, but I have no idea what it may be.– StephenSSep 26, 2020 at 1:07
It's probably down to knowing what the abbreviation represents, like The World Health Organisation - some things are more likely to take an article, because they represent a specific group or institution. Sep 26, 2020 at 16:03