Queue a single lot in the queue and observe that your fill is exactly when you expect. If they were generating fake trades it would be obvious since there would be trades that don't lead to you getting filled when you should be getting filled.

If instead they were actually wash trading on the real market (not just generating fake trades, which is the most common approach), that would be much harder to detect with any kind of forensic analysis of the data, you'd need some whistle-blower. However, doing this would be impossible in a liquid asset like BTCUSD or LTCUSD.