A hack or lifehack should be non-obvious. Otherwise it's not a hack but common sense.
There is no clearly defined border, and to make matters more ambiguous, you can't always tell if there is a hack by just reading a question, because by definition hacks are non-obvious, because you or I are not able to think of a hack, doesn't mean one doesn't exist.
This makes LifeHacks different from sites like StackOverflow, where it's easier to determine if a question falls within the scope of the site...
Also remember that people can leave comments even on closed questions and can request the question to be re-opened. so if you do think of a super new cool hack for a closed question, you can always do that.
We should take care that:
- Questions aren't too broad;
- A good question might attract a lot of obvious & common-sense answers, the problem here is with the answers, not the question.
Specifically, the examples you give:
- "What is the best method of cleaning up broken glass?" might be better off as "Is there a way to pick up broken glass without cutting yourself?" or something to that effect, since the former is too broad.
- "Check for electricity in a socket without tools?" seems too obvious; but perhaps someone has a good answer (unlikely), and they can always request to re-open a question.
- "How to put a slipped bike chain back on the freewheel without getting dirty?" seems okay to me, although the answer is perhaps "too obvious"...