A military recruiter determines if the candidate is a possible recruit. A recruiter will ask about marital status, health, education, drug use, and arrest record. It is important for the candidate to be upfront and truthful when answering questions. Once the recruiter has determined the individual is qualified for additional processing, the ASVAB is scheduled. A physical examination may also be conducted at the time of the test. 

Registration for the ASVAB is done entirely through your local recruiter. They will not want to start the application process without being confident that you will be a qualified applicant, so they will first ask you some information about your age, fitness, and marital status, among other things. They will likely also administer a short practice exam before taking you to a testing center to either give you a realistic idea of which jobs you would qualify for or to help you decide if it would be better to study before taking it.
The underlying model used for the ASVAB is a three-parameter logistic (3PL) model. This model represents the probability that an examinee at a given level of ability will respond correctly to an individual item with given characteristics. The characteristics used are difficulty, discrimination and guessing (this refers to the likelihood that a very low ability examinee would respond correctly simply by guessing).
Don’t be fooled by the appearance of the low numbers, the algorithm that computes the AFQT is very sophisticated. Just because your ASVAB score is 40 doesn’t mean that you only got 40% of the questions correct, it could simply mean that you lack strength in one area, but succeed with high praise in another. Your ASVAB score is only a means by which you are categorized for available positions; it is by no means a measure of intelligence.
The Armed Forces Qualification Test (AFQT) is used by all of the Services to determine if an applicant is eligible for the military. Four of the ASVAB subtests are combined to form the AFQT. It measures general cognitive ability and is composed of verbal and mathematics subtests. AFQT scores are grouped into categories for reporting purposes. The table below shows the AFQT categories and the percentile score ranges corresponding to the categories. Applicants that score in AFQT category IIIA or higher may qualify for enlistment incentives.