Echos can be found as you explore the planets in the game. They are a glowing, blue-ish color and resemble the Secrets also found in the game, but less intense. IGN has a great guide for echo locations throughout the game based on planet:
https://www.ign.com/wikis/star-wars-jedi-fallen-order/Force_...If video guides are more your style, MrVuule has a guide of all locations in one video: