Her suspicion was initially aroused when Jenner and her partner Travis Scott began making public appearances together, and so she posted a video on TikTok stating her theory - just so it remained there as an 'I told you so' if her theory proved to be correct.
Speaking to BuzzFeed News, she said: "I made the video so I could jokingly be like I told you so to friends with a timestamp and not expecting anyone to see it.
"I had like less than 50 followers at the time."
In a subsequent video, she expanded on her theory, explaining that the tell-tale sign was the 24-year-old's nails.
More specifically, it was the fact the nails hadn't changed, which led Tiffany to conclude Jenner had taken a load of photos for social media all at once, so people wouldn't notice any bodily changes.
Tiffany, from Los Angeles, said: "Being the detective that I am, you have to follow patterns.
"Kylie has a pattern of always changing her nails. This is not common for her [not changing her nails], so there's a reason.
"Also, her last pregnancy she stopped posting body pictures completely, which was like so out of the ordinary. That's all she used to post.
"It was so obvious, so this time she's trying to be less obvious."
Well, that prescient video has been viewed 3.4 million times, and many people think Tiffany - who now has 22,800 followers on TikTok - should get snapped up by the FBI.
O'Neill isn't ruling it out - particularly if they have a dedicated pop-culture department.
She said: "I've seen a few people trying to take credit, but I know I was the first person to call out the nails that started all the other inconsistencies in her Instagram presence.
"I wanted the right source credited because I was proud of my investigation skills.
"I like pop culture and I've always wanted to be a detective, so maybe I'll continue combining those two interests."
She also hopes Kylie holds no grudges about her breaking the news two months before she did.
She said: "I hope I didn't ruin anything for her."
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