A handprint is a remnant of alien power use in all versions of the Roswell series. Most notably, Max leaves a handprint mark on Liz after he heals her. A handprint left on the body is often the only evidence of direct alien contact.
Kyle Valenti later notices the handprint on Liz's abdomen when her shirt rises slightly at school, and he tells his father. Sheriff Valenti takes Liz to a morgue to question her about her alien contact, showing her the dead body of a man with a handprint mark on his chest. Valenti believes that the man was killed by an alien. When Liz asks Max about it later, Max tells her that the man died of a heart attack at the mall, and Max unsuccessfully tried to save his life.
To throw off Valenti's search, the group sets up a fake attack with Alex Manes. Alex is "attacked" and his throat is cut, but he's then healed, and the handprint is apparent on his throat. When Sheriff Valenti arrives, Alex tells him about the "stranger" who healed him, sending Valenti chasing off after a truck.
Max Evans leaves a silver handprint on Liz Parker when he heals her. Conversely, Nasedo left a silver handprint on James Atherton and all of Nasedo's subsequent victims. In the pilot, Michael used silver paint to fake the handprint on Maria's body when they attempt to trick Sheriff Valenti into giving up his suspicions about Max.
Sheriff Valenti has a photograph from 1959 of the handprint left on a dead body. His father believes it means the victim was killed by an alien.
Roswell, New Mexico (2019)
Max Evans leaves a silver handprint on Liz Ortecho when he heals her, but it takes a few hours to appear. The handprint creates an empathic psychic connection, and Liz receives an "echo" of Max's feelings. Max believes her feelings for him are just an echo of his feelings for her, so he respectfully refuses to kiss her until the handprint fades.
After Rosa Ortecho's death, a handprint appeared over her mouth, showing that she was killed by an alien. This was covered up by Project Shepherd, and her official autopsy was replaced with a fake.
Noah Bracken uses his powers to absorb his victims' energy for himself, killing them and leaving a handprint on the body. Project Shepherd has been tracking the deaths by the handprint marks left on the victims. After Noah tries and fails to kill Liz, he leaves a handprint on her chest and she receives an echo of his fear.
Liz asks Max to intentionally place a handprint mark on her, so they can experience each others' feelings while making love, and he does so. Through this connection, Liz later feels Max overexert his powers, and it also allows her to track his location.