Normally it's moulded into the gelcoat on transom starboard side. Usually near the waterline, so it is in the hull not deck moulding. Some builders also insert a plate into the moulding - Fairline do it in the base of a port side wardrobe, for example
When buying or checking a boat always keep your antennae on max gain and look for other signs. It is pretty easy to mould in a new HIN to a transom, such that you can't tell it's not the original. I've seen a HIN retro moulded in to a finished boat, personally, with my own eyes and you can't tell