Foundation for testimony
When a witness testifies about something without personal knowledge of the underlying facts we say the testimony lacks foundation. Under Federal Rule of Evidence 602, "A witness may not testify to a matter unless evidence is introduced sufficient to support a finding that the witness has personal knowledge of the matter." Conclusory observations, without the underlying facts are no good. United States v. Dotson, 799 F. 2d 189, 192-93 (5th Cir. 1986). Just saying that he has known the fact for a long time does not establishthat the witness has personal knowledge of a fact. It does not show why or how the witness knew the fact. United States v. Sandles, 469 F. 3d 508, 515 (6th Cir. 2006).
If the proponent of the evidence can't establish a proper foundation, then, on objection, the trial court should exclude the evidence.
Remember to make your adversary lay a proper foundation, and object if he or she does not.