Welcome to the forum.
Niceic assessments (presuming this is for registration as a competent person for Part P self certification) are in place to assess if you are aware of the requirements of the building regulations and have the competence to be able to undertake electrical installation safely and in accordance with the wiring regulations and the building regulations.
If you are a competent electrician you should not have a problem with the assessment.
There are quite a number of threads on this forum covering assessments, try searching for Part P assessment or any of the self certifications schemes and assessment to get a variety of views on the subject.
Essentially the testing is fairly straightforward, they will not be looking to trick you or misdirect you they are only checking your knowledge, competence and organisation. They will want you to pass as it is easier for them and provides them more money but they do have to be competent themselves in the assessment so you do need to know what is what.
You will need to read the welcome pack and ensure that you have in place and ready to demonstrate all the required paperwork and reference books: e.g. qualifications, installation certificates, health and safety working procedures, complaints log, invoicing, BS7671, OSG, GS38, Approved document P (and others), etc.
Always ensure that you are aware of the requirements for both the building regulations and wiring regulations but do remember that any points can be looked up in the reference books that are to hand, it is not a memory test.
Ensure that you have jobs available and accessible to show the inspector. Practise the practical aspects of testing and ensure you know why you are testing and what sort of results you are expecting; if you can be familiar with the demonstrated installation so you can test confidently at the time of the assessment.
Make sure the demonstrated jobs are up to scratch and if there are any items that you feel may be questioned have the answers and regulations prepared to back up your choice.
In general there will be fairly clear cut questions about the requirements of the building regulations not only Part P but also the other aspects of the building regulations that are required, some questions on the limits and requirements of the wiring regulations, Zs values are common and earthing and bonding requirements but it could be any number of things however they are generally asking for facts and not calculations.