There are several ways for the MCU Skrulls to be identified, even when they're masquerading as someone else. Marvel Studios introduced the Skrulls to the MCU in 2019's Captain Marvel, which saw the shape-shifting race seeking refuge after being forced into a lengthy war with the Kree. In a major change from their Marvel Comics history, the MCU's Skrulls were shown as allies to humanity, an otherwise-peaceful race who had forged themselves into warriors to survive. Even so, Phase 5's Secret Invasion has proven how dangerous Skrulls can be, as a rebellious faction has been quietly infiltrating Earth for a number of years, assuming the identities of high-ranking MCU figures.

https://screenrant.com/how-identify-skrull-mcu-secret-invasion-clues/