When? I'm not good with remembering stuff...
I figured in an earlier review or beta. I give it out as a lesson to a ton of people because everyone and their mothers have trouble recognizing when they're about to create a run-on. If I haven't, then to be honest, that's something. (As in, seriously, most of my reviews mention it, which leads me to believe either that the comma is a really tricky bastard or that other schools' English programs just really sucked in comparison with mine.)
Basically, the period test goes like this:
1. When inserting a comma into your sentence, mentally (or physically, whichever works for you) replace it with a period. If what you get as a result are two complete, coherent sentences (as in, you can read them without any context, and they have a subject, predicate, and all the formalities a sentence needs to be a sentence), then you're about to write a comma splice and need either a period or a conjunction there.
2. If you're about to write a conjunction (and, but, or, for, nor, yet, so), do the same thing. Some people don't realize run-ons can be created by linking independent clauses (bits that can serve as their own sentences) with conjunctions and continue into what looks like a compound sentence. (Alternatively, they don't continue into a compound sentence but don't realize they sort of need a comma to make the linked clauses work.)
3. Same thing with semicolons, except semicolons are generally misused all the time, so my advice here is just avoid using semicolons until you can find a reliable guide to teach you the rules.
3a. Colons as well, for that matter. Seriously, guys, as pretty marks of punctuation as they are, please go memorize the rules for using them before attempting to use them.
There's probably something else here that I'm forgetting, but I'll think of it in the morning, probably.
Edit:
What is a good example of a sentence you can write up now that would make the best scholars cry?
Buffalo buffalo Buffalo buffalo buffalo buffalo Buffalo buffalo.
If only because it's technically playing by the rules. Technically.