Look at a piece of the code that interests you. If a couple thousand people each look at a piece of it, any backdoors will probably be found. So not everybody has to look at it, and nobody has to look at *all* of it, but it's still pretty safe.
Do you look in every sourcecode in every program you compile? who says that opensource can't contain backdoors?
No, but I don't run software from untrusted groups that have their reputation at stake if they were found to be hiding backdoors.
We know Eggdrop is 100% backdoor free -- there is the odd bug that turns into a remote exploit (not sure when the last one was .. must be long ago) -- but eggdrop is backdoor free but who knows about windrops .. would be an easy target for someone to pollute.