You need to determine how wide the sterile zone is around your alcohol lamp or Bunsen burner. If you feel heat, then you can be fairly certain that there is an updraft that prevents dust from settling in/on your culture.
I've wondered how to tell that actually. I have been working as close to the flame as I can. Even ended up singing the hair off my arm once, though I was not burned.
I'll admit: I also haven't tried because I haven't just been interested in using the yeast I have banked!
I banked two strains as "practice": dregs from a Sweetwater beer (Their white IPA, which uses a "Belgian Strong Ale" yeast, and Wyeast 3787.) This was to get some experience and see how things went before trying to bank the WL835 that I ordered from the WL vault.