The primary way ECNs determine if a liquidity taker's flow is 'toxic' or not is by looking at aftermath charts. The aftermath chart shows the average mark-to-market profit of trades done by the liquidity taker as a function of either time or number of top-of-book updates (optionally broken down by currency pair). The trade profit is usually viewed from the liquidity provider's perspective, and so the aftermath chart is typically equal to half the average bid/ask spread at the inception of the trade, corresponding to the y-intercept of the aftermath chart.
Now, the time it takes for the aftermath curve to decay to zero represents the average amount of time a liquidity provider has to monetize the spread earned at inception after trading with the given liquidity taker (by matching the trade with another liquidity taker, for example). If this amount of time is relatively short (typ. a few seconds), then the flow is harder to monetize. Such flow is considered 'sharp', and very sharp flow is considered 'toxic'.