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.