How can I be sure that my SL is in the right place?

I like to use some derivative of the ATR (average true range) indicator when determining my SL. ATR measures volatility and automaticaly adjusts my SL to account for that volatility. Using a hard rule like "5 pip" or "10 pip" stop loss is foolish since volatility will change over time. A stop loss rule like 0.5 * ATR(14) pips or 1.5 * ATR(14) pips is so much better.

Trendlines are the best way to place your SL/TP orders ... but be sure that your trends are analysed well ... like having more than two points inorder to confirm a solid trend and to find S/R perfectly ... one more thing ... check your trend on different time frames