What does Watson suspect regarding the telegram?

Watson believes the test with the telegram was worthless.

First of all about the test telegram, which you sent from London in order to make sure that Barrymore was really here. I have already explained that the testimony of the postmaster shows that the test was worthless and that we have no proof one way or the other. I told Sir Henry how the matter stood, and he at once, in his downright fashion, had Barrymore up and asked him whether he had received the telegram himself. Barrymore said that he had.