You can use this simple front-end to query the API. The pull down menu to the left of the search bar has 4 different types of queries you can do: search by Gene ID(s), search by full text query, get the database metadata, and get the available fields in the database (the last two don't accept any input, just click search).

The Return Fields input allows you to specify which fields to return in your variant object. If you are only interested in certain databases, you can choose to return only them. The textbox autocompletes with all available fields.

For more informations and docs, check out the API landing page.