Generally speaking, FCA-regulated broker profile will be found at the Financial Services Register page on FCA website. Try to find it and check if it will match the info from the broker website.
The detailed steps are as below:
1. Find the licensed/reference no.(preferred) or name of the forex broker, which you can get from the broker's website;
2. Enter the reference no. or name into the search bar on https://register.fca.org.uk/s/;

Note:
If you can't find the broker's profile on FCA wesbite, it's probably an unauthorised broker and please keep away of it.
3. And you'll get the broker's contents on FCA website. At this time you will click the 'What can this firm do in the UK?' and check if the broker has the authorization to provide 'Rolling spot forex contract' to 'Retail' customer;

4. If all the steps above have been done, don't forget to check the most important information: check if the firm details published on FCA website matches the ones you'll go to trade with, especially the website and email etc.
If no, please keep away the broker because it's probably an unauthorized firm and your money will fall in danger.
