When I lock one I write a post and put "LOCKED" at the bottom of it (usually with a short explaination, like this thread is crappy/old/needed to be euthanized years ago/etc.). I say usually that's what I do, but truth be told my head is so big (sure it is Rare :sleep_125 ) that I've locked a mamoth 2 threads so far. Some mods might not tell you who locked them, or if they were deleted you'll never know. Suffice to say that the mods probably won't lock anything that is (1) aviation or military related, (2) not dissolved into a flame war, or run it's course, or (3) not already asked and answered in another thread.
Of course, I can't speak for everyone, and every case might be different, but this place is intended to be primarily Aviation related (see the welcome message ...).