When my daughter was assessed for it they used a machine thingy where she had to watch something on a screen and respond to it while her eye movements and focus were being measured over a prolonged period of time. It ruled it out for her, which I knew it would. She has AS so has the opposite problem attention surplus, to the exclusion of all else.
I am not so concerned about the attention side of ADHD, but the hyperactivity- from my brief google research it seems they can have either or both set of 'symptoms'. My dd has all the hyperactivity signs in lists given on the internet (clumsy, inappropriate climbing, runs all the time, competitive, talks constantly, interrupts etc) Ive always thought she is just 'high energy' but its seems to be escalating and also becoming more 'inappropriate' IYSWIM. The school have already said she is dyslexic. I know she is very enthusiastic about school and tries hard. She is told off for talking in class but other than that they have never said anything about her behaviour
She was being assessed for that but they also had to test for ADHD because they can over lap. For example, my daughter is also clumsy, competitive, talks constantly, interrups and got in trouble in school for fiddling. She doesn't have conversations with people, she talks at them, a wall of noise.
So really the only way to tell is to trust your instincts, and if you are concerned get her assessed.
My dd is 7 she has ADHD mainly hyperactive an impulsive. The paed said difficult to dx in girls as they have better coping strategies. She was diagnosed over the course of a year with myself and the school completing several Conners questionnaires on her behaviour several times a year
It can be very difficult, especially in a child of 6 years. You aren't meant to diagnose ADHD below 7 years as the behaviours can be normal.
The criteria for diagnosis are the DSM IV (mainly used in the uk), and there are lots of screening tools you can use to help assess, like the Connors questionnaire.
If you are worried speak to your GP, they can refer you to a local community paediatrician who specialises in behavioural medicine (asd and ADHD). The assessments can take a long time (cause it's difficult and you need to make sure you're right!), so if you're really worried I'd get a referral sooner than later.