To check if your data has been breached under GDPR, you should check to see if you've received a data breach notification letter. This will confirm whether or not your data has been breached. If you don't have one of these but suspect your data has been breached, you should contact the company you believe is responsible and ask them to investigate. If you'd like to learn more, we recommend checking out our detailed guide here https://www.legalexpert.co.uk/data-breach-compensation/