To check if a car is taxed in the UK, the simplest and official method is to use the free online service provided by the UK Government. Here is the step- by-step process:

1. Visit the "check if a vehicle is taxed" page on the official GOV.UK website.
2. Click "Start now."
3. Enter the vehicle's registration number (license plate) and click "Continue."
4. Confirm the vehicle details shown are correct; if so, confirm to proceed.
5. The website will show the vehicle's current tax status at the top of the page. A green status means the car is currently taxed, whereas red indicates it is not currently road legal.
6. Additionally, the page may show the vehicle's MOT status if relevant.

This process provides the most up-to-date and legally accurate confirmation of whether a vehicle is taxed or not. Using just the vehicle registration number, it can be done quickly online for free. There are also other tools and websites that provide similar checks but GOV.UK is the official source.

In summary, checking your car's tax status in the UK is easiest by using the GOV.UK vehicle tax checker online with your car's registration number.