Side quests are marked on your map with large orange question marks. They are pretty hard to miss. Just make sure you visit all the restaurants in the game and choose the gossip option so all the hot spots on your map are marked for you. Once you remove the fog of war from the map you can see the question marks if you haven't taken a quest yet. Just complete 80 of them and the achievement is yours.
Many of the side quests are follow up quests to other side quests as well. I know Lestallum has a lot of chain side quests.
Once you get far enough into the game you will get the option to use Umbra to travel to the past. You can use him by going to a bed to sleep in and choose Umbra instead of yes or no for the option to sleep. When in his menu, choose the top option to go back to the past and you can complete any quests that you haven't completed yet. This is also how you access the quest to find Adamantoise once you are far enough in the game. I went back after finishing chapter 13 and got the quest to find the turtle. You can probably get it sooner.