Here is a useful little label that you can print out and stick on a box that you don't want accidently turned over. You can see that not only do I have an arrow pointing up and designated accordingly, I have another appropriately marked arrow pointing down. For purposes of conciseness, I have used this handy abbreviation, dn.

Now, you see, if the box were upside-down, it would be immediately apparent, because the label would look like this!