Because people are generally better at detecting mistakes in others’ work than in their own, a prudent principle is that one should always have one’s own work checked by someone else.
Principle
Since it's easier for you to see mistakes in other people's work than in your own, it's a good idea to have your work checked by someone else.
Evaluate / Goal
This isn't quite a conditional principle, but we still want to look for a scenario that seems to apply to the first half and then deliver the second half.
i.e. we want a situation where it would be easier for you to see what's wrong with someone else's than with yours, and so you ask someone else to check yours out.