Best way to tell without a professional:
Browse ADHD memes together.
If she relates to the memes far more frequently than you, she might have ADHD.
If you both relate to the memes in equal frequency, I've got some bad news.
Sidenote:
People with ADHD tend to be friends with people with ADHD

Pretty common experience, that I've even seen memes of people discovering they're ADHD from memes.
Note: not everyone has the exact same experiences, so nobody with ADHD will relate to every meme.
But there are patterns, and if youre being honest with yourself, its the memes relating to thought patterns and internal dialogue which will be the most telling.