Courts can use circumstantial evidence without direct evidence so it would be up to a court to determine if the circumstantial evidence was strong enough without having actually prove the use.
Circumstantial evidence can support a finding that a communication was intercepted, even absent direct evidence
("[D]irect evidence of a fact is not required. Circumstantial evidence is not only sufficient, but may also be more certain, satisfying and persuasive than direct evidence.").
GS2