Just Enough Advancements (JEA)
An addon for JEI which let you search for advancements. This is pronounced like "yeah".
An addon for JEI which shows Advancements in JEI. You can search for them as they were items. They will tell you what you need to do to reach them, even if they aren't unlocked!
You can click them to see a description that explains what you need to do in order to for them.
As the client normally only knows about advancements already completed, the mod is also required on the server.