If its a new house, then surely a steel lintel is far more likely? They are not easy, having found this the hard way myself. It may be a case of drilling through the plasterboard and seeing what lies beneath. If a steel lintel then the appropriate drill (i.e. a normal HSS
or cobalt drill) will do the biz, but if its a long way back, then a conventional plug may be tricky. Options may include putting the curtain rail right at the top of the wall above the lintel, so you're into conventional blockwork, but in many case the lintel is high enough that this isn't an option.
if it is a concrete lintel, then as said before, an SDS drill is just so much better and less dramatic than a conventional hammer drill. Less 'orrible bloodcurdling noise, too!
If its a timber framed house, then.... umm, find out how its built!