The Sounds of Poetry: A Brief Guide, Robert Pinksy
https://www.amazon.com/Sounds-Poetry-Brief-Guide/dp/0374526176
A great all-purpose introduction to how poetry works, why poetry is written the way it is, and what the point of it all is. Written by the former Poet Laureate of the United States who is also well-respected for his translations of Dante.
The easiest way is to say it out loud. Most words are pretty obvious. For instance say syllable stressing the first syllable first (SILL-ubble) and then say it with the second syllable stressed (sill-UB-ull). Finally say it with the last syllable stressed (sill-ub-ULL). Usually only one will sound right. It isn't always super clear, but most words will be pretty clear (PRI-tee/pri-TEE, SOO-per/soo-PER). There's always variance in the amount of stress given to a sound, and Pinsky argues that there is no true Spondee in English, (a unit of two stressed feet/sounds) but there is always one syllable with more stress than the other. Scancion isn't an exact science, but reading aloud, exaggerating each sound is the easiest way to tell in my experience.