Serious coders are always publishing on GitHub or make a program’s code visible as a part of open source act.
Because we are no coders, we need to rely on educated coder reviews and opinions of experts. If in doubt, we can ask a question here on Bitcointalk and a coder will look into it.
(Almost) every coder knows about GibHub and can access it as well.
FWIW, just asking for coder or anyone who can code probably isn't enough. I don't expect backdoor which intentionally added or security issue on low-level programming level can be discovered by average coder.