Though a graphic accent can tell you exactly how “e” and “o” are pronounced (è = open “e,” é = closed “e,” ò = open “o”), if there is no accent, the rules to determine open versus closed vowels are very complex.
The good news is that even if you pronounce an “e” or “o” wrong, according to the Tuscan standard, your Italian will still be perfectly understandable by everyone. In fact, most Italians do not know or apply these rules, and just use open/closed “e” and “o” according to their regional customs.