Paul,
The number one way is when the source code is built is it correct. Did it build correctly and is it functioning as designed and according to the requirement specifications.
For me personally I actually train my developers to check in code from wherever they want to. However, I warn them, it has to be able to be built, if not I will tell them to fix it. A few times of doing this and not meeting schedules if applicable should take care of any cowboy coders out there.
Regards,
Joe