There's two major tests for the efficacy of depression-treating drugs that I am aware of. One is a "willpower" test, which in one incarnation is attempting to drown a mouse. A compound more likely to be efficacious in humans if it makes a mouse attempt to swim longer before ultimately succumbing to exhaustion. Taken by itself this test seems promising, though one could argue it might blunt whatever mechanism makes you give up and try a different approach.

The second is a bit worse, and done in humans. A compound more likely to be efficacious in humans at treating depression also seems to be effective at disrupting short and long term memory. So, they give the sample and a short term memory test and see how you do. If you do worse it is likely to treat depression.

You will see, if you go back through past threads, people stating that their ADHD diagnosis was comorbid with or developed after a diagnosis of depression. It is extremely likely that they developed focus problems due to being treated with SSRIs.