They can't exactly bust an athlete with the bio passport unless some levels are off-the-charts above certain thresholds; however, a knowledgeable person can pretty quickly determine if an athlete is "likely doping" (the leaked documents showed us this) and then they can develop a pattern of testing to try to catch them or at least give an athlete an idea that they need to cut it out (I think USADA is more likely to do this than to try to catch anyone). AIU got Shelby because they did two tests in quick succession within a few days of each other, and that pretty much never happens. Shelby thought she had the green light after the first test and then likely got caught because she took something the night before her failed test.
Updated USADA leaders in numbers of tests this year in case anyone was wondering:
12: ShaCarri Richardson (hahahahaha!!!!!)
11: Sean McGorty
10: Kiera D'Amato, Natosha Rogers
9: Anna Hall, Christian Coleman, Clayton Young, Elle St. Pierre, Fred Kerley, Nick Christie
8: Erriyon Knighton, Evan Jager, Galen Rupp, Grant Fisher, Hillary Bor, Hobbs Kessler, Karissa Schweizer, Miranda Melville, Payton Otterdahl, Quincy Hall, Sara Hall, Vashti Cunningham, Zouhair Talbi
Others of note: Noah Lyles (7), Sydney McLaughlin-Levrone (7), Yared Nuguse (7), Nia Akins (7), Nikki Hiltz (7), Ryan Crouser (6), Cole Hocker (6), Elise Cranny (6), Chase Jackson (5), Kenny Bednarek (5), Valerie Allman (4), Rai Benjamin (3)