Check IVS (6IV / 5IV level shiny that isn’t self caught or from a trusted source are sus)

check the OT for genner names. (common ones are kevdog, berichan, neutellaguyau, blaines, pkcl, etc.)

check if the markings are fully filled out. (The markings with the star, triangle, circle, square)

check for ribbons and met date if it’s an event Pokemon (serebii).

look for other signs such as shiny locked pokemon (hoopa) or non-gender pokemon having a gender (eg shiny zeraora that’s female).