If a trade is closed by Stop loss, the volume is doubled; if it's closed by Take profit the minimum volume is used. OnTradeTransaction is used to determine whether a trade was performed after the activation of Stop loss or Take profit.

The excellent enumeration ENUM_DEAL_REASON was added in build 1625:

ENUM_DEAL_REASON	Reason description
...	...
DEAL_REASON_SL	The operation was executed as a result of Stop Loss activation
DEAL_REASON_TP	The operation was executed as a result of Take Profit activation
...	...

It can be checked in OnTradeTransaction.

In other words, it is a very simple and reliable way to determine that a trade resulted from Take Profit or Stop Loss.

At the moment (build 1626), this Expert Advisor can only be checked in a live test - by launching it on a chart or in a debug mode on real data (F5 in the MetaEditor editor). I used the following bypass maneuver:

In OnTradeTransaction, we determined whether Take Profit or Stop Loss triggered:

      if(deal_symbol==m_symbol.Name() && deal_magic==m_magic)
         if(deal_entry==DEAL_ENTRY_OUT)
           {
            if(deal_reason==DEAL_REASON_SL)
               ExtLot*=2.0;
            else if(deal_reason==DEAL_REASON_TP)
               ExtLot=m_symbol.LotsMin();
           }