A number of online tools are available for you to get a reasonable idea if your symptoms are consistent with adult ADHD. These tests alone are not diagnostic of ADHD!
If you decide to seek a diagnosis through the NHS you will need some form of initial screen before you speak to your GP (usually the ASRS v1.1) so these tests do have value.
ASRS v1.1 (explained in the glossary here)
An online version of the ASRS v1.1 can be found here. The first six questions in this test can predict ADHD with sensitivity above 90%.
ADHD Symptom Spectrum Checker
The ADHD spectrum encompasses a range of symptoms. Not all of them are included in a medical diagnosis from a psychiatrist, but they can still be problematic. As there is considerable variation in the type and severity of the symptoms, this tool can often be useful as it allows you to visualise your symptoms.