Electricalserv.
I will try to answer for Elecsa, but it is not definitive.
The assessor will check you have,
BS7671 Big Red Book, any other books are useful, and although not a requirement, he will expect to see an OSG.
Public Liability insurance certificate.
Qualifications, minimum 17th Edition.
Memo of the EAWR 1989.
Approved Document P (download version is acceptable)
H&S policy.
Complaints procedure.
Risk assessment.
Appropriate Test Instruments, WITH current calibration certificate.
Completed Electrical Installation Certificates, Minor Works Certificates.
The questions asked will depend on the type of job you are using for the assessment, and there are thousands he could ask. you wont be expected to know all the answers, but you will be expected to be able to find out using the books.
He will ask you to perform a couple of tests on the assessment job, these will depend on the circumstances if it is work in progress, or if the property is occupied or not. He will almost always ask questions about the earthing arrangement of the installation, and supply characteristics. If it is not practical to do a certain test, then he may ask you to describe in detail how you would perform a test like a Ze, and what the expected result would be.
The assessor's are not there to catch you out. All you have to do is prove competence, and that you can work safe, and to a good standard, you don't need to be perfect. If you do that you will have no problems.
Cheers..........Howard