The answer is cargo samples, particularly manifold samples.
When faced with a contamination or off-spec claim at the discharge port, the onus is on the vessel to show that the condition of the cargo has not changed between loading and discharge, and properly taken load port samples can provide such evidence. Joint sampling is preferred, however if it cannot be arranged then samples taken by crew are equally beneficial.
Remember - a pro-active approach to sampling can potentially save millions of dollars in claims and prevent delays to the vessel.
To learn more
Insight Liquid cargo sampling
Gard Guidance to Masters 18.104.22.168 Cargo samples