Before you engage in talks with a development firm, it is a best practice to sign an NDA, or Non-Disclosure Agreement.  This document protects both you and the development firm and allows you to freely talk about your ideas and lets the development firm openly discuss their development practices and any proprietary development tactics or software that they have.  Any competent developer should have this document readily available however you may want to also have a lawyer review this document before you sign it if you are truly concerned about it.

The best way to ensure that you own the source code for your app (or website) is to make certain that the company your working with has that specifically spelled out in the contract terms. A trustworthy company should not hold either hostage for a royalty.

MAKE SURE YOU CAN WALK AWAY WITH YOUR SOURCE CODE AT ANY TIME

Unfortunately, I see this happen way too many times, from individual investors to fortune 500 companies I get calls on a monthly basis from frustrated people who did not review their contract for specific language giving them total ownership of their IP and source code.