Simple. Put an order to slap the ask and watch L2 at the same time. If it's fake, you will see that their wall will disappear momentarily and reappear.
I just saw it happen with whoever hit the ask with 200,000 shares (shout out to that person!)
Mostly you guess or get a feeling by the way the stock has been trading. The only way that I know of to "test" it is to place a buy order for much of the wall and see if that much goes away.