Warning: contains spoilers for Supergirl: Woman of Tomorrow #3!
In Supergirl: Woman of Tomorrow #3, DC reveals the one power that Supergirl and Superman struggle to use on aliens! Kryptonians possess a wide range of powers, but they don't work absolutely anywhere. There are limitations to their superhuman abilities. For instance, under a red sun, Kryptonians are weakened and slowly lose all of their powers due to their cells being drained of yellow solar energy. But in Kara's case, it's not that her powers are being blocked, but that they're more complicated than they seem.
Supergirl is on the hunt across the universe with her new companion Ruthye. After denying Ruthye's request to kill her father's murderer, Supergirl was struck with tragedy. The villainous Krem attacked them out of nowhere and poisoned Krypto the Superdog. This led Kara to accept Ruthye's original offer, as she needs information from Krem to help save Krypto. It's been made clear that Supergirl might go as far as to kill Krem, but there's still a chance that she just brings him to justice, without crossing her usual moral guidelines.
In Supergirl: Woman of Tomorrow #3 by Tom King and Bilquis Evely, Supergirl and Ruthye arrive on an alien planet named Coronn. It turns out they have been there for over three weeks searching for leads on Krem's whereabouts. Supergirl approaches a very uninviting alien receptionist in search of past planetary records. The alien is very dismissive and lies about seeing or hearing about Krem's arrival and Supergirl knows it. However, it's not because she used her super senses like a lie detector - a favorite Kryptonian trick - as she informs Ruthye that even after a few weeks on an alien world, a new species' physiology is too hard to read.
Normally, Superman and Supergirl would be able to listen to a person's heartbeat to see if they were lying. But thanks to the different physiology of the alien species, Supergirl isn't able to draw on this trick. It's a reminder of how differently the Kryptonian heroes experience the world - Superman and Supergirl are usually able to discern with their powers whether someone is lying to them (unless that person has specifically trained to avoid this), but out in the wilds of space, Kara doesn't have this option. Thankfully, the alien is a bad enough liar that she can tell anyway.
Supergirl is certain about the alien's deception, highlighting the fact that one of her greatest strengths in this series is simply her worldliness and experience saving lives and stopping evil all over the galaxy. It turns out that when Supergirl or Superman touch down on an alien planet, they never know if they might be about to lose the ability that allows them to sense deception, and so as on Coronn, they have to fall back on the much more human ability to assess the people in front of them and decide who deserves their trust.