trigger accessing data in another table before firing in home table

[Steven DeMonnin]

I am just studying the docs and learning by working the docs and out of books so forgive if the question is silly.

I have a customer table for a media loan app.
in this table I have things like name, address, etc and a column for amounts owed and a column that is set when the amount owed is greater than zero. but my checkout procedure does not access those columns so the item just is allowed.

I have another column for particular media item which includes a column for which customer has that item.

(Item id, title id, who has the item's customer id)

I have a small procedure that just inserts the customer id into the particular media item table.

Can I have a trigger on the table for the particular media item that checks the customer id table to make sure they don't owe money first before it allows the update to media item table?