How to know a word's proper pronunciation without ...

Well, I can tell you what I (lifelong native speaker) do: I read it over a few times, look at the context for meaning, take a flying leap based upon what I know about etymology, and then guess.

I'm right about 60% of the time, though when I'm wrong I can be spectactularly wrong. I have an advantage over many native speakers in that I studied Classical Latin and Homeric Greek in high school, and many of our borrowed words have Latinate or Romance-language roots.

There are basic rules of English phonetics and orthography that will get you a long way towards 'mostly right' when pronouncing words unheard. Some of the rules fail for early, Anglo-Saxon derived words because the way they were written down was 'correctly' phonetic to the writing system in the region of Britain where they were first written down, although the spoken version of English that ended up dominant was from a different region with different pronunciations. This variation between how it was first written down and how it's pronounced today is where all the trainwreck for words spelt with a 'gh' in them comes from (thought/through/cough/right), as well as a lot of our vowel randomness.

A lot of the other failures of 'standard' phonetic English choices come from borrowed French words, which we either a) both pronounce and spell mostly like the original French word, b) change the spelling to make it phonetic by English rules but still pronounce it the same way, or c) keep the spelling and change the pronunciation to make it a bit more 'regular' feeling in English.

There are tips and tricks for figuring out on the fly where a word is likely to have come from, especially with French-looking words. Words with consonant clusters unusual in English (pizza, pharmacy, extraction, blintz, gherkin, tsunami, et multi alia) also have a little signpost of their origins.

The thing that will trip you up worst is the emPHASis you put on which syLABble, because sometimes we preserve fossil emphasis patterns even when we reprounounce the word, especially with proper nouns.

Take a flying leap at it, but be willing to be corrected. And eventually your vocabulary will be big enough that any genuinely new words you're encountering are probably used only rarely. :->

For example, I was a very widely-read child with an enormous written/read vocabulary, but many of the words I knew from books, I'd never heard pronounced, and I didn't bother to look them all up in phonetic dictionary listings. So I knew the word 'ribald' and knew what it meant just fine. Then I encountered Tom Lehrer's satirical song 'Smut' and he uses a word that I heard and mentally spelled 'ribbled,' because he rhymes it with 'quibbled' and 'nibbled'. Turns out that's how you say 'ribald'. :->

It occurs at about 1:17 in this video, if you're curious. Not precisely work-safe, but not graphic either.

Lehrer is amazing at wordplay and rhyming, twisting words all out of shape when necessary, and he delights in rhyming words that are pronounced the same but spelled utterly differently (like viewed/lewd in this same song). Every once in a while he mispronounces a word outright, but usually there's a pause or a grin or facial expression that makes it clear he knows he's being 'naughty' with his English.