It’s called the DSM-V, and it has an entire article about Bucky in it.
Not by name or anything, but Steve knows how to read between the lines. Some of the criteria are things he’s been aware of since they were kids, and some of them are bright moments of realisation as memories of Bucky’s various quirks are dragged into a whole new light.
People with ADHD show a persistent pattern of inattention and/or hyperactivity-impulsivity that interferes with functioning or development…often fails to give close attention to details…difficulty sustaining attention in tasks…easily distracted by extraneous stimuli…fidgets with or taps hands or squirms in seat…blurts out answers before questions have been completed…
Steve thinks the DSM-V would have been handy to have around when he and Bucky were in school, back when the only behavioural distinction their schoolmasters drew was between boys who sat still through their lessons and boys who would very soon wish that they had. By some mysterious convergence of natural genius and sheer good luck, Bucky’s school reports always came out with glowing grades - accompanied by bewildered and often acerbic commentary on his many failings of discipline. “He is a spirited young lad,” said one of the kinder masters. “His marked moral defect causes disruption to classroom order and to the progress of his fellow students,” wrote one of the less kind.
“You should read this,” Steve tells Bucky, handing over a stapled photocopy of the relevant pages - if they look more like a dossier and less like an actual book, Steve figures Bucky might actually pay attention instead of shoving them under the bed with the rest of his unsorted laundry and broken household furnishings.
“Sure,” says Bucky, and…doesn’t. As if the universe is determined to support Steve’s suspicions with as much evidence as possible, he promptly loses the print-out. Steve sets aside the temptation to feel frustrated and just reads the article aloud over a nice strong cup of coffee.
When he’s done, there’s a long moment of silence broken only by the rhythmic drumming of Bucky’s fingers on the table. “Well, shit,” says Bucky thoughtfully. “That’s…I dunno, I thought I was just lazy or something.”
He sounds perfectly nonchalant, but Steve knows better. “You’re not lazy,” he snorts, and leans over the table to give Bucky’s “You’re the hardest-working guy I know.” He takes in the barely-concealed look of skepticism and discomfort and hope on Bucky’s face and knows that there’s not going to be a conversation about this - not yet. Bucky has spent too much time and effort burying this to open it all back up now.
Later, though, the DSM-V goes missing. Steve finds it in the same place he finds Bucky: up on the secluded roof in a patch of warm afternoon sun, drumming his fingers unconsciously on the tiles as he pores over the book with his tongue between his teeth and a deep furrow of concentration on his brow.