In particular, I'm trying to insulate my loft. I'm not keen on the idea of taking a naked flame up there!
You can have a blower-door test done (or rent a blower and do it yourself). Basically you close all your doors/windows, and then stick a big fan in your doorway (sealed), then have it blow air outside. It creates negative pressure inside your house, amplifying any air leaks. I had this done as part of an energy audit (and when it was fairly cold outside), and just walked around using my hand to feel for any leaks (they are pretty obvious, because it is like a jet of cold air shooting in).