The legal term probative describes something that tends to demonstrate or prove something. A weapon with the accused's fingerprints on it would be considered probative evidence at a trial.
In the law, the phrase "probative value" is used a lot, generally meaning "the ability of a piece evidence to prove something important in a trial." Probative comes from the Latin probativus, "belonging to proof," and is commonly understood among lawyers and judges to mean "tending to prove." Did you manage to get a photo of that lady in the act of stealing your neighbor's dog? That's definitely probative evidence!