>>10106311
He doesn't draw knees. There are a FEW characters that have their knees bent in their portraits, but the actual knee portion is blocked by their clothes or covered (Kogasa). One of the few portraits that shows a bent knee with that resembles or implies a kneecap is Youmu's in PCB, while Ichirin in UFO does not, even though she shares a similar pose.
But in all seriousness, it's the eyes. He draws irises more circular than oval. Also, the skin colors of most of the characters is very pale (looks a little sickly), which started becoming the norm back in MoF. Some characters have very distinct differences that break away from normal ZUN art, such as Marisa having significantly less prominent highlights in her hair in UFO and TD compared to the rest of the cast, along with Yoshika.
Because he probably never learned or put much time into learning to draw "properly" and there is no hint of any sort of references used, it would be really hard to perfectly nail down his art style, and he seems to be getting ready for another shift judging from Youmu's TD art.