Free margin in Forex is the amount of money that is not involved in any trade. You can use it to take more positions, however, that isn't all - as the free margin is the difference between equity and margin. If your open positions make you money, the more they achieve profit, the greater the equity you will have, so you will have more free margin as a result. There may be a situation when you have some open positions and also some pending orders simultaneously.
Back tests – You can read my forex robot reviews to see if the forex robot has back tests which will give you a good idea how it performed historically, some forex robots even back test as for as 15+ years! Ideally, you would want back tests to have been done using real tick data and spreads, thus making the forex robot back test results as accurate as possible in the mt4 strategy tester