[russellbeattie]

> "...which side of the whole deck is the 'top'..."

A dark line drawn across the top of the deck would be enough. Though it would ruin the stealth factor of the cards.

Also, the pattern on the back of some playing decks isn't symmetrical, so that could be used as well.

[istjohn]

Decode it both ways and see which isn't gibberish.

[OkayPhysicist]

Use a casino decommissioned deck. They typically have either a hole punched in them, or a corner cut off. Either way it won't be symmetric, but still perfectly plausible as a cheap deck of cards.