I remember the general rule as described in the book 'Swan's Practical English'. It says...
Adverb particles can go between the verb and the subject noun.
Switch off the light over Switch the light off
Having this said,
check out the source code - is fine and looks a bit better
Now, why check the source code out does not sound okay?
Well, I had asked this question here (though not able to trace) and the crux I got from the answers by natives is splitting the adverbial particle from verb does not sound natural as the length of the sentence (clause) between them increases.
This is the reason, for longer sentences, it's better to keep check out as check out
Check out [longer phrase] - looks natural
Check [shorter phrase] out - looks natural
Though some may call it grammatical, but to others (including me!) it does not sound natural.