At some point in the survey process (it can differ at the request of buyer or surveyors), your yacht is going to leave the dock and head for sea trial. This will be where all mechanical systems are going to have to function in a real-world environment. Engines, transmissions, motor mounts, generators, stabilizers, steering, navigation equipment will all be stressed and tested.
If an engine survey team has been hired, they will perform additional testing focused on the yacht's engines and generators in conjunction with your hull surveyor who will be focused on navigational and other systems during sea trial.
There is no a doubt what will be asked of your vessel at sea trial. The yacht will need to run properly at all RPM band ranges, from idle all the way up to wide-open throttle. Saying, “I never run my boat at WOT" is not an option or excuse - as it will be asked to be proven. Does your yacht make full RPM without vibrating or overheating? If the answer is, “yes,” and those three things pass, you have fewer worries. Should it not make full RPM, or vibrates or overheats, buyer concern and post-survey issues will be a hurdle to cross.
Oil samples are commonly taken which are sent for independent lab testing at expense of the buyer.