Check your business and personal reports regularly, and always before you apply for finance. Looking at your own file is a soft search and does not affect your score. Reviewing it early gives you time to spot errors, understand what an underwriter will see, and get anything wrong put right first.
Sensible trigger points are before any funding application, after settling or closing an account, after a change of address or registered details, and any time you are declined unexpectedly. A decline you do not understand is often explained by something visible on the file.
Look for accounts you do not recognise, balances or statuses that are out of date, addresses that are not yours, and public record entries such as judgments. If you are a sole trader, remember your personal and business files are separate records and a lender may look at both, so check both.
Experian, Equifax and TransUnion each hold their own data and use their own scale, so a good result at one does not guarantee the same elsewhere. All three offer statutory access to your report, and each also offers its own free and paid options that change over time — go to the agencies direct to see what is currently available rather than through a third party promising something more.