Phones are locked if you buy them with subsidies from the network provider, otherwise they are unlocked.
Where did you buy the phone? Did you pay the full retail price, or get it as part of a deal?
If you paid the full price, then it's definitely unlocked.
If it's locked, you can easily check if the second SIM slot is locked simply by borrowing a SIM from a friend on a different network, or by buying a PAYG SIM (I imagine they are available for 5 Euros) from a different network.