I don't think that one can try and create a test for LGBT representation to mimic tests for gender representation.
When it comes to gender representation, it's easy for a character's gender to be identified because of nonprimary sexual characteristics. You can show a character with a beard, or a character with breasts, and we can deduce that that character is male or female without being shown a penis or vagina. When creating a character, nonprimary sexual characteristics are (almost) always something that is included, and it's extremely difficult to have a character that gets any substantial attention without including nonprimary sexual characteristics of that character (and by extension, gender). I believe that a large factor in why female/male representation of characters is skewed is due to the nonprimary sexual characteristics of males and females being such that the male characteristics are the ones that can be hidden/addressed more easily - that is, the leap from no sexual characteristics to male is smaller than the leap from no sexual characteristics to female.
When it comes to sexuality, then there aren't any nonprimary characteristics. Unless we're examining the character's sexual interests then the character's sexuality isn't relevant to anything. Sometimes characters are attempted to be given nonprimary characteristics of their sexuality (e.g. making a gay man extremely flamboyant) but every instance of this that I know of this is badly done. Attempting to declare a character's sexuality through nonsexual means indicates that the creator of the character is trying to make the character's sexuality relevant to something that it isn't, or that the creator feels that the inclusion of a character representing that sexuality is more important than narrative consistency. There's also a much stronger case for heterosexuality being the norm, given sexuality demographics and that for the vast majority of characters the question never has to be addressed.
I would put Dumbledore as a great example of a gay character done right. Dumbledore was an incredibly important character in the HP universe. He had a complex backstory, had interesting qualities and flaws, and had meaningful interactions with a lot of the other characters. But most critically, of the seven books, it's not until a good portion into the last one that his sexuality is revealed. At no point did Rowling think "the readers need to know Dumbledore's gay", so the matter of his sexuality didn't come up until we were learning about his backstory and it was relevant. When it did come up, it wasn't treated as a big reveal that shocked everyone, nor was it unnecessarily referenced for the rest of the book.
If you try to examine various works of fiction to investigate the sexuality of those involved, then you run into the problem that, for the vast majority of characters, sexuality is never investigated. Typically, the only characters whose sexuality is investigated is the protagonist and the interests of the protagonist, and the protagonist is affected by a lot of factors that other characters wouldn't be - for example, making the typical consumer identify with the protagonist my making the character as similar to the consumer as possible has an enormous effect on how well the consumer appreciates that character and the work the character is in (an example is the twilight books, where the main character Bella is given extremely few identifying characteristics (I believe that this accounts for a lot of the protagonist gender skew, as "male" is seen as a lot more nondescript than "female"))