I've figured out how to determine if a UF/PNR novel is YA or not just by looking at the cover.

If there is a female featured on the cover with her back to the viewer, it's adult.

If there is a female looking super sad at flowers or some shit, it's YA.

If the female on the cover is wearing leather, it's adult.

If the female on the cover is wearing what looks like a Victorian nightgown, it's YA.

If there is a vampire biting a female's throat and there is blood visible, it's adult.

If there is a vampire biting a female's throat and there is no blood visible, it's YA.

On a side note, if there are boxing ring ropes on the cover, it's angsty NA.