If your income is above the relevant threshold HMRC will write to you, confirming that you must start using Making Tax Digital for Income Tax by the start of the upcoming tax year.
For example, if HMRC review your return and find your qualifying income for the 2024 to 2025 tax year is over £50,000, they will to you and confirm that you must start using Making Tax Digital for Income Tax by 6 April 2026.
It is still your responsibility to check if and when you need to use the service, and make sure you are signed up and prepared to use it when you need to.
If you do not receive a letter but think you need to sign up because your qualifying income is over a certain amount, you can:
use the HMRC tool to check if you need to sign up
speak to your tax agent, if you have one