So i'm about to start running a campaign where my group will start out in a city that is led by a king who happens to be a Great Wyrm Gold Dragon in alternate form. I'm playing it with a whole back story that ties in with the city, but it works where no one in the city knows what the king actually is. One of my players is playing with a type of dragon template for his character, which i've nerfed for the most part, but is still technically making him a dragon. So my question is this: can a dragon, by any special power or natural ability, recognize another dragon that is in humanoid form and has done nothing that would suggest he might be a dragon? More specifically, will my player be able to tell what the king is if he comes within his vicinity or in contact with him? I'm not a top dragon buff, but i've done some research on them and can't find anything that says he would be able to. That being said, i'd prefer to not go in thinking that he can't and it actually turns out he can. Any help on this matter is greatly appreciated.
By default, no
That is; Dragons (or any other creature types that I know of) have no default abilities that allow them to see the creature type of any other creature type, nor can they sense transmutations or illusions, they don't have Truesight (unless it says so in their entry, but I don't think any Dragons have it by default), so they don't have any special information.
But Dragons are old, powerful and usually paranoid
So any specific dragon might very well make sure that he can spot other creatures being disguised or trying to fool him. Good odds that a Great Wyrm will have an item that gives him True Seeing permanently to spot other trying to trick him. Although this still does not let him see a creature's Type, it will show him other dragons in hiding.