If you have a small ad spend but good size market you should consider only running ads on the Discovery network, which is Google controlled inventory, i.e. no fraud.

Otherwise, as sprfrkr said, you need to stay on top of your placements and exclude any that have super high CTRs and super low CTRs. There are some scripts and tools around for this be at my agency we prefer to do this manually so we can ‘see’ what’s going on.