This question is going to be a bit difficult because dictionaries use a wide range of “standards”. Some of them use the International Phonetic Alphabet, some use Phonemic Alphabets and some have come up with their own systems to varying degrees of success. In most dictionaries, there is a page or two at the beginning that explain the structure of its entries. They should mention there what system is being used or whether they created their own.
If you want to learn more about the International Phonetic Alphabet, you can find a guide here: International Phonetic Alphabet and Phonemic Alphabets
 Merriam Webster uses IPA – https://www.merriam-webster.com/assets/mw/static/pdf/help/guide-to-pronunciation.pdf
 Cambridge Dictionaries use a modified IPA – Help – Phonetics
 Oxford Dictionaries use IPA as well – Pronunciation guide for English and Academic English Dictionaries at OxfordLearnersDictionaries.com