Evidence collected by private investigators need to be relevant, obtained legally and unadulterated.
There are 2 categories of evidence: Direct and Indirect. Direct evidence proves a fact without any presumptions or inferences and can either be direct testimony of a witness, item or document.
Indirect evidence (also known as circumstantial evidence), in the absence of direct evidence, proves a fact by inference from circumstances using reasoning and experience. Such evidence can be in the form of fingerprints, surveillance videos, audio recordings, etc.