How do I Know when a Dictionary is Loaded?

Use the Ajax object which has the condition "On completed", which is triggered when its finished loaded.

https://www.construct.net/en/make-games/manuals/construct-3/plugin-reference/ajax

or you could do;

A new system action Wait for previous actions to complete. This is a new kind of wait action, similar to Wait 1 second, but instead waiting for any actions that run in the background to complete. For example the AJAX Request action triggers On complete when it finishes, but now you can put Wait for previous actions to complete after the Request action and then use the data right after that. This means you can do the whole AJAX request in a single event, and you don't need to worry about tags either.

AFAIK, Load from JSON is not an asynchronous operation. It is completed in the same tick. Even if JSON string is very big, there will be a lag, but the next action will not be performed until data is loaded into the dictionary. So you don't need any wait at all.

Asynchronous actions are marked with a little "clock" icon in the event sheet. When these actions are finished, they usually trigger "On completed" event, or you can use "Wait for previous action to complete" as mOOnpunk suggested.

All other actions are completed immediately and you don't need to use Wait.