NIM39205 - National Insurance numbers (NINOs): confirming a NINO: confirmation by individual - form CA5403
You should check the other guidance available on GOV.UK from HMRC as Brexit updates to those pages are being prioritised before manuals.
Customers can find their National Insurance numbers on payslips, P60s or letters about tax, pensions and benefits. If they have a Personal Tax Account they can also view it there and print a confirmation letter. The quickest way for customers to get their National Insurance number is through logging into or creating a Personal Tax Account or downloading the HMRC app. Their Personal Tax Account allows customers to get confirmation of their National Insurance number, download a letter confirming it, which can be printed off or saved and share their number with employers.