You could store the trades in 2 arrays, one for 'previous tick' trades and one for current trades. This will tell you that your order has just closed.
You could then examine the closed trade and compare the closeprice with your stoploss. If the closeprice is equal to, or worse than your stoploss then it can be assumed that the trade was closed by your stoploss.
I reckon this method might not be totally infallible but I would say it should be 99% accurate.
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