I'm guessing this is because of the Fate/stay re-awakenings. With those I went by who it couldn't have been at first, and that would be the original three artists. It's obviously not UCMM or Natsushima, and Ichinose (sadly) is too busy for DG.
That just leaves Shiratori, Hyōe, and Kitano.
Like how you can tell it's UCMM from the bottom eyelashes, and Ichinose from the circles in the eyes, Shiratori’s telltale sign is the way they shade/color skin, it’s not how the other artist do it. I don’t know any other way to describe it, other than it’s flat when in juxtaposition to the other artists. Compare Helen and Melete with Rin, Saber, and Archer. But, Gilgamesh could be either Kitano or Hyōe because it’s more vibrant and there’s more shading involved.
Hyōe is a special case, they improved rather quickly so I thought they were a separate artist at first. And I don’t remember when they started changing up their style, so at first they could be confused with Shiratori.
There’s also an image floating somewhere online that listed each artist + series (including some collabs), it seemed pretty accurate. I’ll try to find it, could’ve sworn I saved it. In the end it's hard to tell with collabs in general because they tend to try to copy the artstyle.
Ah I see! Thanks and right Shiratori's shading is easy to recognize, but Hyôe and Kitano... From the comments on Forte's illustration when he was released, I was sure it was Kitano who drew him, but it was Hyôe? I really can't tell the difference between their artstyles...
For Forte I went by what people were saying on twitter, but now looking at him he can definitely be drawn by Kitano. His eyes resemble Kitano's style more. And if he is, then I think we can say Gilgamesh is also drawn by Kitano because they both have hard shading in their muscle definition and slightly defined bottom lips.
I checked to see if maybe Kitano made any comments about Forte but all they said was they got and evolved him. Regardless, I'm gonna switch him to Kitano.
Kitano's skin coloring is also more blended and detailed (Ariton, Viviane Fatima), Hyōe's art is unblended (Strom, Riina, Ein). Which makes me think Hyōe drew Mutton because I don't see that in Mutton.