Put simply there is no official way to do it. It was quite common 25 years ago to do a club check and sign it as P1/S, nothing said that it had to be a licencing prof check, that came much later.
From Art 103 the Law says:
(4) The information recorded in accordance with paragraph (3) must include—
(c) the capacity in which the holder acted in flight;
(e) information about any test or examination undertaken by the holder of the log whilst in
flight.
So long as you have fulfilled the requirements of Art 103 you are OK
It is a personal flying log book, you can put in it what you like, if its subsequently not acceptable for some purpose then sobeit.