Before I adopt a child is there a way to test a 1 or 2 year old for ADHD like with a cat scan, MRI or any other way?

No. Adhd is a clinical diagnosis, based on symptoms of inattention, behaviors and performance. There aren't any imaging tests that can diagnose it and currently there are no criteria for diagnosing it in 2 year olds.
ADHD is real. Adhd is well described deficit of cortical function. It is neither behavioral or educational in nature. Although mri, fmri and other research tools have proven beyond doubt that adhd is a real disorder, these tools aren't readily available for clinical use. 1 or 2 year old brains are sufficiently under developed that a diagnosis is not usually possible. Adhd usually presents around 4 yo.
Tests for ADHD. There are several available tools (like a stethoscope) that can indicate the presence of adhd. Two highly regarded and statistically validated tools are the tower of london (drexel) and the peeramid-2/peer-2/peex-2 series from mel levine, md. Additionally, several physical examination signs are easily demonstrated in adhd patients including mixed dominance, extraocular dysmetria, and synkinesia.
No. There is no objective test for adhd. The diagnosis is made from a checklist of subjective symptoms. That is why I do not consider it a real medical diagnosis. When i look for the underlying medical or educational cause of the adhd symptoms and treat them, most of the time, they resolve.