Warning: spoilers ahead for the twelfth episode of Supergirl Season 3, called "For Good."
Season 3 of Supergirl has been building Reign to become Kara's most dangerous enemy of the series to date. As a Worldkiller of Kryptonian origin, Reign has a lot of Kara's powers without some of her biggest weaknesses. She also has a major advantage on the good guys of National City in that they still have no idea who she is. Fans have known all along that Reign is the alter ego of none other than the lovable single mom Sam, but everybody else -- including Sam herself -- has been blissfully unaware. In last week's episode, Sam's double life began to take its toll, and she started to unravel to Alex. In "For Good," Sam and Alex teamed up to try and figure out what was going on, and their story may have revealed how Sam's secret identity as Reign will be exposed.
Alex got to use her science smarts rather than her sidearms in "For Good" when she began to run all manner of tests on Sam to try and determine what could be causing the blackouts and memory loss. An MRI revealed that nothing abnormal was going on in her brain, and the end of the episode proved that none of the other tests showed anything indicating a tumor or anything else. While Alex, Kara, and Lena were overjoyed at learning Sam won't be dying of a brain tumor and orphaning poor Ruby, Sam was still terrified at the fact that she didn't know what was wrong with her.
The tests obviously weren't abnormal enough for Alex to think that Sam is anything other than 100% human, but there's a big chance that one of the tests conducted on Sam in "For Good" will come back in a significant way later in Season 3. The camera lingered on the scene in which Alex drew blood from Sam, which could indicate that the blood sample will make all the difference in the mystery unfolding.
Unlike the MRI and other imaging tests, the blood sample could yield answers about Sam's biology. After all, the MRI just took pictures of Sam's brain. The blood could theoretically be tested in some very specific ways, with or without Sam's cooperation. If Alex becomes suspicious that not everything is as it seems with Sam, she could do further tests on the blood, and those tests could yield the evidence that outs Sam as Reign.
At this point, however, Alex and the others may have bigger fish to fry than what's going on with Sam. The end of "For Good" revealed that we're going to be getting a good look at another Worldkiller sooner rather than later, and the last thing National City needs is a second Worldkiller joining Reign. The priority at the DEO might be containing Purity over hunting and/or figuring out Reign. We'll have to wait and see. Hopefully the Legion of Super-Heroes will stick around to help.