I'm in between jobs in my field (unrelated to software development), and I recently picked up a temporary side contract writing a few applications for a firm. I'm the only person working on these specific applications. Are there ways I should be checking myself to make sure my applications are sound? I test my code, try to think of edge cases, generate sample data, use source control, etc. but since I'm the only person working on these applications, I'm worried I'll miss bugs that would easily be found in a team environment. Once I finish the application, either when I'm happy with it or when my deadline expires, the firm plans to use it in production. Any advice? Not to use a cliche, but as of now, I simply work "to the best of my ability" and hope that it's enough.

Incidentally, I'm under both strict NDA's and laws about classified material, so I don't discuss the applications with friends who have actually worked in software development. (In case it's not obvious, I am not a software developer by trade, and even my experience with other aspects of information technology/computer science are limited and restrained to dabbling for the most part).